VNC is completely cross platform capable. VNC servers and clients are available for most major operating systems: Win32, Mac OS, Linux, BSD and any other obscure modern platform.
This means that you can run a VNC server on a Windows machine, then remotely control it using a Mac. The same Mac can open a simultaneous VNC connection to a VNC server running on a Linux box. This makes VNC quite a flexible tool for remote administration.
Note that on most open source platforms, each instance of VNC server (using vncserver) creates a completely independent GUI environment. You have to use x11vnc to make the on-screen GUI environment the VNC server. However, on Windows platform, a VNC server typically only use the on-screen GUI environment as the source.
This is not a technical deficiency. Rather, it is a licensing issue and the restriction is artificial. Without this restriction, a relatively resourceful machine can serve a number of users at the same time, using only one installation of Windows. Of course, this cuts into the profit of the publisher.
I have heard rumors that Windows XP Home actually permits up to three simultaneous GUI environments (desktops), but have not checked the validity of these rumors.