VNC

The venerable Virtual Network Computing that allows remote access to desktops across platforms (Linux, Windows, . Works very well, especially if used within a compressed ssh tunnel.

NX

Have not looked into this but it sounds interesting! (nyt)

RDesktop

Client for Microsoft's proprietary RDP protocol (nyt)

Related

• Nice comparison on Wikipedia

Items marked with (nyt) indicate I have not yet tested it. I've heard
it's good (so it has made it on the list) but have not yet tested it
myself.