Answer:
   
   Most people are careful to translate their Hebrew and Yiddish, but a
   few are common enough that people don't bother. Credit goes to
   Meredith Warshaw for compiling much of the list below. [H] indicates
   Hebrew, [Y] indicates Yiddish.
     * Agunah (literal translation: chained).
       A woman who cannot remarry; usually because her husband refuses to
       give her a get (divorce), because there is no way to verify
       whether or not he is dead, or because he is incompetent to give a
       divorce (i.e., mentally ill). [H]
     * Assur
       Something prohibited. [H]
     * Averah (aveira)
       Sin, transgression of G-d's will [H]
     * Baal Tefillah
       Prayer leader [H]
     * Ba'al Teshuvah (literal translation: master of return)
       A penitent; a Jew who returns to a traditional observant Jewish
       lifestyle (also known by the acronym BT) [H]
     * B'chor
       Firstborn status [H]
     * Bamidbar (literal translation: in the desert)
       Numbers (4th book of the Torah) [H]
     * Bet Din
       Court of Jewish law [H]
     * Bikkur Cholim
       Visiting the ill or hospitalized [H]
     * Bracha (pl. brachot)
       Blessing [H]
     * B'reshit (literal translation: in the beginning)
       Genesis (1st book of the Torah) [H]
     * B'rit milah
       Covenant of circumcision [H]
     * B'shaah tova
       Congratulations to an expectent mother (literarily "in a good
       hour," means "at an auspicious time," i.e. may whatever time your
       child is born be a good time.") Also the correct response to
       announcement of a marriage engagement. In both cases, it is in
       anticipation of a "mazel tov" for something hoped for, that has
       not yet occurred. [H]
     * Chag sameach
       A happy holiday (used as a greeting) [H]
     * Chayav
       One who is obligated (chiyuv=obligation) [H]
     * Chazzan (hazzan)
       Cantor [H]
     * Cherem
       Excommunication (from cessation of aid, boycott) [H]
     * Cheshbon Hanefesh (Literal translation: accounting of the soul)
       Self-examination of your actions' merit, or accounting of your
       soul's good and bad aspects. [H]
     * Chevra
       Friends; comrades [H]
     * Chevra Kadisha (Literal translation: holy society)
       The group that prepares a body for burial. [H]
     * Chillul Hashem
       Desecration of the divine name [H]
     * Chok (pl. chukim)
       Law from the Torah deemed to be without a humanly-discernable
       rationale, e.g., the red heifer. [H]
     * Chometz (chametz, hametz)
       Leavened food, which is forbidden during Pesach [H]
     * Chumash
       The five books of the Torah, bound in one volume (not a scroll)
       [H]
     * Chumra 
       Stringency -- custom of a community to observe more strictly [H]
     * "Dati/lo dati" 
       Dati = religious, lo dati=not religious, as used in current Hebrew
       in Israel, but it is a black and white distinction, meaning
       Orthodox and not Orthodox. [H]
     * Daven
       Pray (from Yiddish, with a particular emotional sense) [H]
     * Derech Eretz
       Respectful, menschlich, considerate of others [H]
     * Din
       Law, judgment [H]
     * Drasha
       Interpretation of a Torah passage (often a creative
       interpretation) (from a root meaning "search") [H]
     * Dukhn
       Perform the kohen's blessing before the congregation [H]
     * D'var torah (pl. divrei torah) (Literal translation: word of
       Torah)
       A Torah discourse, homily or sermon [H]
     * D'varim (Literal translation: words, things)
       5th book of the Torah (Deuteronomy) [H]
     * Emet 
       Truth [H]
     * Emunah 
       Faith [H]
     * Frum 
       Observant (often with a right-wing Orthodox implication). Derived
       from the German/Yiddish word for "pious". [Y]
     * Gemara (Literal translation: learning, from the Aramaic)
       The later part of the Talmud, which expands upon the Mishna [H]
     * Get (pl. gittin)
       Document of divorce [H]
     * G'milut Chasadim
       Acts of loving kindess [H]
     * Hachamim (chachamim)
       Sages [H]
     * Haftarah
       The selection from the book of prophets read after the Torah
       reading. [H]
     * Halacha (Literal translation: path)
       Jewish law [H]
     * Halbanat Panim (literal translation: whitening the face)
       Causing someone to blanch by public embarrassment [H]
     * Hashgacha 
       Ritual supervision, most often used in terms of kashrut/dietary
       laws, although it can also refer to spiritual or moral supervision
       as in a yeshiva or dormitory [H]
     * Heksher 
       Kosher certification [H]
     * Hesed (chessed)
       Kindness [H]
     * Heter
       Permission (usually a rabbinic ruling that permits something) [H]
     * Hiddur Mitzvah
       Beautifying physical objects involved in a mitzvah, or otherwise
       adding to a mitzvah an esthetic sense [H]
     * Kabbalat ol Mitzvot (literal translation: acceptance of the yoke
       of the commandments)
       Acceptance of commandments as binding [H]
     * Kabbalat Shabbat 
       Service welcoming the Sabbath [H]
     * Kavanah 
       Intention, devotion, inner concentration during prayer [H]
     * Kevah 
       Fixed; a fixed time; fixed words or prayer (often contrasted with
       kavanah, inner concertration during prayer) [H]
     * Kiddush Hashem 
       Sanctification of the divine name; martyrdom [H]
     * Kiddushin 
       Betrothal (for the purpose of marriage) [H]
     * Klal 
       A general principle [H]
     * Klal Yisrael 
       The Jewish community as a whole [H]
     * Koach 
       Strength [H]
     * Kohelet 
       The book of Ecclesiastes [H]
     * Kol Hakavod (literal translation: all honor)
       Used idiomatically to express praise or congratulations for an
       achievement [H]
     * Kol Isha 
       The voice of a women (considered by the Rabbis of the Talmud to be
       distracting to men and thus lewd). [H]
     * Kol Tuv 
       Everything good (may you be blessed with everything good) [H]
     * Kulot 
       Leniencies [H]
     * K'vod Hatzibur 
       The honor of the community [H]
     * L'shon Hara (Literal translation: "evil tongue")
       Defaming or badmouthing [H]
     * Leyn 
       To read (usually to read Torah) [Y]
     * Maariv 
       Evening; the evening prayer service [H]
     * Machmeer 
       Stringent; one who observes a chumrah (stringency) [H]
     * Maftir 
       The aliyah consisting of the last few lines of the Torah reading,
       or the person assigned that aliyah. The person assigned the maftir
       aliyah also chants Haftarah. [H]
     * Mamzer 
       A person born from a prohibited union (i.e., from an incestuous or
       adulterous union) [H]
     * Mara d'atra (literal translation: master of the place)
       The local rabbi, whose decision carries the force of law in that
       locality [Aramaic]
     * Mashgichim 
       Ritual supervisers of kashrut who watch/supervise on the premises
       for dietary supervision of ingedients, food preparation, serving,
       dishes and cutlery, etc. [H]
     * Mashiach (Moshiach) 
       Messiah [H]
     * Mechitsa 
       Division; a barrier separating men from women in the synagogue [H]
     * Midrash 
       An interpretation; a story that fills in gaps in the Torah
       narrative, or answers questions about the narrative; (when
       capitalized) any of several volumes of such stories compiled by
       rabbis of the Talmudic era [H]
     * Mincha 
       The afternoon prayer service [H]
     * Minhag 
       Custom [H]
     * Minhag ha-makom 
       Local custom [H]
     * Minyan 
       Quorum of 10 needed for a public prayer service. In traditional
       synagogues, only men over the age of 13 are counted towards a
       minyan. Liberal (non-orthodox) congregations also include women
       over 12 in the count. [H]
     * Mishna (Capitalized)
       The early core of the Talmud, consisting primarily of case law
       decisions. (Not capitalized) one unit, typically a single sentence
       or short paragraph, in the Mishna. [H]
     * Mishpat (pl. mishpatim)
       Law from the Torah that can be rationalized [H]
     * Mitzvah 
       Commandment; not "good deed" in Hebrew, but has come to mean that
       in Yiddish, especially among more secular people [H]
     * Mutar 
       Permitted [H]
     * Ner tamid 
       Eternal light [H]
     * Nigun (pl. nigunim)
       Wordless prayer melody, usually repeated many times over to create
       a spiritual mood [H]
     * "Nu?" 
       This is an exclamation used in the same sense as "well" "eh" and
       "hey." [Y] It could be used in the Hebrew/Yiddish translation of
       any of the following:
         1. Well, do you want the egg roll or the knish?
         2. Hey! Stop throwing paper airplanes in class.
         3. My experimental tofu-liver-garlic cholent tastes good, eh?
         4. So, Becca, I hear you and Izzy went out last week. Well?
         5. A rebuke (on small kids): "Nu, nu, nu, you spiled all the
            milk!
         6. To express doubt: "I heard that Rabin met Asad. Nu."
         7. When the news ain't new no more: (see #7; the change is in
            the tone of the "nu").
         8. As "come on": NU BEMET.
         9. When one can't talk (i.e. in the middle of Shmone-Esre, after
            Netila before Hamotzi, etc.)
     * Olam ha-ba 
       The world to come [H]
     * Parsha (N)
       The weekly Torah portion (pl. parshiot) [H]
     * Patur 
       Something or someone who is exempt (from an obligation or a law)
       [H]
     * Pikuah Nefesh 
       To save a life (usually in context of breaking Shabbat, etc.) [H]
     * Posek (pl. poskim) (N)
       The rabbi one consults for halachic decisions; an authority on
       Jewish Law [H]
     * Posken (V)
       To render an halachic ruling, usually one that clarifies the law
       in a specific case [H]
     * Psak (N)
       Decision, verdict [H]
     * R'chilut 
       Gossip [H]
     * Ribono shel Olam 
       Master of the universe [H]
     * Rosh Chodesh 
       First day of the new Jewish month [H]
     * Ruach 
       Spirit, wind [H]
     * Sedra 
       The weekly Torah portion [H]
     * Shachrit 
       Morning; the morning prayer service [H]
     * Shaliach 
       Emmisary, appointed agent (male pl. sh'lichim, sh'lichei; fem.
       sing. sh'lichah; fem. pl. sh'lichot) [H]
     * Shaliach Tzibur 
       The person leading services [H]
     * Sh'lom bayit 
       Peace in the home [H]
     * Shekhinah (Literal translation: that which dwells)
       G-d's presence (often associated with feminine imagery, but not
       always) [H]
     * Shir Ha Shirim 
       Song of Songs [H]
     * Sh'mot (Literal translation: names)
       The 2nd book of the Torah (Exodus) [H]
     * Shomer (pl. shomrim)
       Watchman, guardian [H]
     * Shomer shabbat 
       Observant of the laws of Shabbat [H]
     * Shoresh 
       Root of a word (all hebrew verbs have a 3-4 letter root that is
       the basis of conjugation. many other parts of speach (adj, nouns)
       are also derived from this same shoresh) [H]
     * Shtiebel 
       A small synagogue [Y]
     * Sinat chinam 
       Gratuitous hatred [H]
     * Taharah (pl. taharot)
       Ritual purity [H]
     * Takkanah (N)
       Correction; a rabbinic edict that supersedes the existing halachah
       (pl. takkanot) [H]
     * Tanach 
       Acronym for Torah Nevi'im K'tuvim - Torah, Prophets, Writings)
       The three divisions of the Hebrew Bible [H]
     * Tana'im 
       Sages of the Mishnaic period [H]
     * Tefila 
       Prayer [H]
     * Teshuva 
       Return, repentance [H]
     * T'hillim 
       Psalms [H]
     * Tikkun Olam 
       Correcting the world, repairing the world; an action promoting
       social justice [H]
     * Torah misinai 
       (Definition: lit, Torah from Mount Sinai)
       Refers to the doctrine that the entire Torah, including the Oral
       Law, was given to Moses at Sinai. [H]
     * Tsniut 
       Modesty [H]
     * Tzedakah 
       Righteousness; used for charitable donations, though the root has
       a very different sense from the root of "charity." [H]
     * Vayikrah (Literal translation: "and He called")
       3rd book of the Torah (Leviticus) [H]
     * Yahrzeit (literal translation: year-time)
       Anniversary of a death; a 24-hour candle lit to commemorate the
       death anniversary of a close relative, also lit on holy days when
       Yizkor (prayer of remembrance) is recited [Y]
     * Yasher koach (Literal translation: meaning unclear, but poss.
       "straight strength")
       Used idiomatically to express praise or thanks for serving in a
       religious or ceremonial role. Implies "may your stength continue,
       go on straight", i.e. "You done good! Do it many times more!" [H]
     * Yotzei (Literal translation: gone out)
       One who has properly fulfilled an obligation [H]
       
   NOTE on spelling/transliteration: Some people transliterate the letter
   "het" as "ch," and others as "h." (Better would be "h" with a dot
   under it, but that's not possible in email. This also ignores the
   linguists, who prefer "x.") Thus: "hag/chag sameah/sameach," "bikkur
   holim/cholim," etc. All the terms beginning with "ch" on the list are
   "het" words.