Virtual Network Computing
Virtual Network Computing (VNC) is remote control software which allows you to view and fully interact with one computer desktop (the "VNC server") using a simple program (the "VNC viewer") on another computer desktop anywhere on the Internet.
The two computers don't even have to be the same type. For example you can use VNC to view a Windows Vista desktop located at the office on a Linux or Mac computer while you are at home. For ultimate simplicity, there is even a Java viewer so that any desktop can be controlled remotely from within a web browser without having to install additional software.
VNC is in widespread, active use by millions throughout industry, academia, and privately. There are several versions to choose from, including a free version and some substantially enhanced commercial versions.
(adapted from RealVNC.com)