Here we get into a real rat's-nest of software. Basically, Linux can run four types of binaries with varying degrees of success: Linux, iBCS, Win16/Win32s (with dosemu and, someday, with Wine), and Mac/68k (with Executor). I'll just discuss native Linux and common Unix software, except to say that WordPerfect for SCO, and quite probably other commercial word processing software, runs fine under Linux's iBCS emulation.
For Linux itself, choices are mostly limited to those available for Unix in general:
Most markup languages are more suitable for large or repetative projects, where you want the computer to control the layout of the text to make things uniform. Trying to make a pretty sign in a markup language would probably hurt...
nroffThis was one of the first Unix markup
languages. Man pages are the most common examples of things formatted
in *roff macros; many people swear by them, but nroff has, to me at
least, a more arcane syntax than needed, and probably makes a poor
choice for new works. It is worth knowing, though, that you can
typeset a man page directly into postscript with groff. Most man
commands will do this for you with man -t foo | lpr.
TeX, and the macro package LaTeX, are one of the most widely
used markup languages on Unix. Technical works are frequently written
in LaTeX because it greatly simplifies the layout issues and is
still one of the few text processing systems to support
mathematics both completely and well. TeX's output format is
dvi, and is converted to PostScript or Hewlett Packard's PCL with
dvips or dvilj.
There is at least one free sgml parser available for Unix and Linux; it forms the basis of Linuxdoc-SGML's homegrown document system. It can support other DTD's, as well.
Someone suggested that for simple projects, it may suffice to write it in HTML and print it out using Netscape. I disagree, but YMMV.
There is a general shortage of WYSIWYG text processing software. Feel free to write some, and do let me know of anything I've omitted here.
LyX is a front-end to LaTeX which looks very promising. See the LyX Homepage for more information.
AUIS includes ez, a WYSIWYG-style editor with most basic word
processor features, HTML capabilities, and full MIME email and
newsgroup support.
At least Caldera and Red Hat ship packages containing the usual office apps like a WYSIWYGish word processor and a spreadsheet. I would assume they do a dandy job, but I've never used them. I think Caldera also ships Sun's WABI, so you could probably run something like MS Office under that if you had to integrate with other folks' files.
Jeff Phillips <jeff@I_RATUS.org> uses Caldera's WordPerfect
for Linux (on Slackware, of all things) and says that it works well.
It apparently includes built-in printer support, as one would expect.
Caldera should have info on
http://www.caldera.com/.
RedHat ships a suite called Applixware; you can find their web site at http://www.redhat.com/.
Other vendors feel free to drop me a line with your offerings.