xditview, a previewer for device-independent troff, is in contrib in X11R6;
it was a supported client in X11R5 and X11R4. X11R4 also offers the
contributed xtroff; an earlier version of xtroff also appeared on the R3
contributed source.

In addition, the xman client can be used to preview troff documents which use
the -man macros (i.e. man pages).

If psroff is used its output can be viewed with a PostScript previewer.

Groff, a C++-based [tn]roff document formatter from the Free Software
Foundation, includes an X-based document previwer based probably on
xditview.  Groff can put out both dvi and PostScript, so xdvi or GhostView
can be used to preview formatted documents.

In addition:

xproof, an X previewer for ditroff has been contributed by Marvin Solomon
(solomon@cs.wisc.edu); version 3.5 is available on ftp.x.org in
R5contrib/xproof*. [8/90]

Elan Computer Group (CA: 415-964-2200) produces eroff, a modified troff
implementation, and Elan/Express, an X11 eroff previewer.

SoftQuad (416-239-4801, USA only 800-387-2777; mail@sq.com) offers SoftQuad
Publishing Software, including a substantially- rewritten troff formatter, a
better intermediate language with backwards compatibility, and an X11[R3,R4]
previewer. (This is the package adopted by AT&T's own MIS department, and
used in and re-sold by many parts of AT&T).  [information from Ian Darwin,
SoftQuad (ian@sq.com) 3/90]

Image Network (1-800-TOXROFF; CA: 415-967-0542) offers the Xroff package,
which includes a fine modified troff implementation and a set of X11-based
page previewers. (This is the package OEM'ed by several hardware vendors.)

[mostly courtesy moraes@cs.toronto.edu (Mark Moraes)] [2/90]