Most of the common Unix tools and programs have been ported to Linux, including almost all of the GNU stuff and many X clients from various sources. Actually, ported is often too strong a word, since many programs compile out of the box without modifications, or only small modifications, because Linux tracks POSIX quite closely. Unfortunately, there are not very many end-user applications at this time, but recently, this has begun to change. Here is an incomplete list of software that is known to work under Linux.
ls, tr, sed, awk and so
on (you name it, Linux probably has it).
gcc, gdb, make, bison,
flex, perl, rcs, cvs, prof.
C, C++, Objective C, Java, Modula-3, Modula-2, Oberon, Ada95, Pascal, Fortran, ML, scheme, Tcl/tk, Perl, Python, Common Lisp, and many others.
X11R5 (XFree86 2.x), X11R6 (XFree86 3.x), MGR.
GNU Emacs, XEmacs, MicroEmacs, jove, ez,
epoch, elvis (GNU vi), vim, vile, joe,
pico, jed, and others.
bash (POSIX sh-compatible), zsh (includes ksh
compatiblity mode), pdksh, tcsh, csh, rc,
es, ash
(mostly sh-compatible shell used as /bin/sh by BSD), and
many more.
Taylor (BNU-compatible) UUCP, SLIP,
CSLIP, PPP, kermit, szrz, minicom, pcomm,
xcomm, term (runs multiple shells, redirects network
activity, and allows remote X, all over one modem line), Seyon
(popular X-windows communications program), and several fax and
voice-mail (using ZyXEL and other modems) packages are
available. Of course, remote serial logins are supported.
C-news, innd, trn, nn, tin,
smail, elm, mh, pine, etc.
TeX, groff, doc, ez,
Linuxdoc-SGML, and others.
Nethack, several Muds and X games, and lots of others. One of those games is looking through all the games available at tsx-11 and sunsite.
AUIS, the Andrew User Interface System. ez is part of this suite.
All of these programs (and this isn't even a hundredth of what is available) are freely available. Commercial software is becoming widely available; ask the vendor of your favorite package if they support Linux.