VNC works great and it's free (http://www.uk.research.att.com/vnc/). You install the VNC server software on the machine you want to control remotely, and you use the VNC client on the machine from which you wish to connect. The same machine can be both a server and a client. VNC is amazingly cross-platform -- you can run the software on virtually any system out there. You can even just use a Web browser instead of the client software if you'd like -- just connect to a host PC with a Java-enabled Web browser using port 5800 to get a Java server that allows you to control the server.  There's even a Palm VNC, which allows you to use a Palm device to control a Windows, Mac, or UNIX machine.

If you're behind a router, you can configure it to send the incoming VNC requests to the proper computer. By default, VNC uses port 5900, and you can just keep adding 1 to the port and have it sent to the next computer (in other words, 5903 would go to one IP, 5904 to another, and so on). You'd want a static IP; if you don't have a static IP, use something like the DynDNS client and a free DNS service to get a domain (we recommend http://www.dyndns.org).