There definitely
are cross-platform remote-control applications. One I know of is VNC. It also happens to have the enormous benefit of being, since it is under the GPL license, completely, 100% legally free.
For OS X I would recommend
VNCThing. If you enter "VNC" while under the Mac OS X tab at
versiontracker you will get
this. VNCThing isn't the most recently made VNC tool, but it worked the best for me out of all of those listed. It has both server and client capabilities.
For Windows or UNIX visit the creators of VNC (Bell Labs, part of AT&T now)
here. They have free client and server packages.
Be warned -- it's pretty slow unless you have a really good connection! I used it to access a friend's old PC at school as a replacement for buying Virtual PC. It was cool but pretty slow. A direct gigabit connection between my G4 and someone's Titanium Powerbook gave a pretty decent speed. But it's fun anyway!
Post if you get it and let me know what you think of the speeds...