There are a few different acronyms that will pop up frequently.

 IMHO:   In My Humble Opinion  (i.e. this is just my opinion)
 IMNSHO: In My Not-So-Humble Opinion
 YMMV:   Your Mileage May Vary (i.e. this works for me, but might not for you)
 BTW:    By The Way
 FYI:    For Your Information
 F'ex:   For example (e.g.)
 RL:     Real-Life
 IC:     In Character [referring to roleplaying]
 LOL:    Laughs Out Loud
 ROTFL:  Rolls On The Floor Laughing
 {g}:    {grin} -- Another system of expressing emotion involves abbreviations
         like this...
 {g,d,r}:{grin, duck and run}