Well, there's a terminal services client (rdesktop.sf.net) but I think that's only useful if you have an NT server to connect to.
For Linux/Other to Linux communication, there are more options (try searching sourceforge.net or freshmeat.net for “remote desktop”). TightVNC I've heard mentioned a few times.
X itself can run across a network (that is, you can run an X application on computer A and have it displayed on a computer B instead; you don't even need a GUI on A) See here:
http://www.tldp.org/HOWTO/Remote-X-Apps.html