X Window, which comes with OS X (normally on the XCode disk).
Try on your Mac's Terminal.App:
ssh -X linuxmachine xterm
(it might be that both your Mac and Linux machine have to have each other's name on /etc/hosts, at least it does not matter).
Oh, I used Leopard to test that. On Tiger, you need to start X and maybe use xterm where you give the ssh command.
The connection seems quite slow. It that matters, you might like to check NX for OSX.