A hallmark of VNC is that it is practically operating system independant:
you can find Servers and Viewers for almost any OS.

Another great asset of VNC is that it is open-source. When the original
AT&T development effort appeared to stagnate (circa 2000-2001), many 
independant developers took up the challenge and created offshoots: new
"flavors" of VNC, optimized in some way different from the original AT&T 
version.

Recently, the AT&T team re-launched themselves as the RealVNC team, and
are again actively developing and supporting the "original" VNC platform.
They do great work, but so do many of the offshoot maintainers. A quick
summary of all known offshoots is here.

Also listed here are some popular 3rd party "VNC Add-On's" that might make 
your VNC experience that much easier, or that much more secure.

List of Known VNC Flavors

Popular 3rd-party VNC Add-on's