| Virtual Network Computing [ From http://www.realvnc.com ] "is remote control software which allows you to view and interact with one computer (the "server") using a simple program (the "viewer") on another computer anywhere on the Internet. The two computers don't even have to be the same type, so for example you can use VNC to view an office Linux machine on your Windows PC at home. VNC is freely and publicly available and is in widespread active use by millions throughout industry, academia and privately."
It is important to note that the free software easily compiles under both UNIX and MS Windows, so one can improve it trivially.
Also several interesting variations exist which increase the security such as SSHVnc, http://3sp.com/catalog/product_info.php?products_id=30 , which automatically makes an SSH tunnel to the server and pipes VNC through the secure tunnel. In this case, VNC is as secure as the current version of SSH installed on the server machine.