Did you know that since version 10.4, Mac OSX has a VNC server built in? It was originally implemented to have remote control and management. Since 10.5 (where Apple Remote Desktop was implemented for remote managing), they have renamed it to screen sharing, but the principal remains the same.
To enable it, follow the instructions below.
The following guidelines are for Mac OSX 10.6, but they are roughly the same for other versions.
To connect to the VNC, you need a VNC viewer. Depending on what OS you're on, you are a bit limited to which client you will be using: