VNC lets you control other computers all over the world: and it’s free!
[Editor's note: This article was updated 18-Nov-2007 to remove URLs that were out of date and directed users to inappropriate sites. For the latest on VNC and remote control software, consult TidBITS articles on Apple Remote Desktop, Timbuktu Pro, LogMeIn.com, and Leopard's Screen Sharing and Back to My Mac.]
I walked into my office and was flabbergasted