What you want is a VNC server and client. There are plenty of these available, as this search will show you:
http://www.versiontracker.com/php/se...macosx&x=0&y=0
My favourite VNC server is OSXVNC. There are also some available that allow you to connect via a web-browser, so the client computer doesn't need to have VNC client softwar installed.
Apple offer their own, very advanced solution called "Apple Remote Desktop". I believe Tivoli also offer a high-end commercial remote desktop solution.