Here are a few of the X client programs that your system may have:
resize: helps programs in xterm windows recognize new window size
xbiff: tells you when new electronic mail comes in
xclipboard: helps with copying and pasting text
xdpr: prints a window (with the printer)
xedit: simple text editor
xmag: magnifies parts of the screen
xman: browses UNIX manual (reference) pages
xmh: electronic mail program
xset: sets user preferences
For more information on those programs, see your online documentation or the O'Reilly X Window System User's Guide.