Controlling a Mac remotely
The Mac comes with the server part of the software built-in for "Apple Remote Desktop"; They can be viewed and managed remotely if you have the client software. For most of us that's too expensive a solution, so an alternate is to use VNC. Check out JollysFastVNC and Chicken of the VNC.

Also, iChat, which comes with the Macs, can do screen sharing, which might be all you need. Check out "Share and share alike." from Apple's iChat site and "Sharing your screen with a buddy".