From the website:

"VNCon is an easy to use remote administration tool which allows you to 
connect, view and take control of any computers running VNC (Virtual Network
Computing) on your network. The list of computers can be obtained using 
various methods, either probing your domain, scan a set of ip ranges, or 
entering each computers details manually."

VNCon is Windows-only and not open-source. The IP scan for VNC is purported
to be very fast and very flexible. You can get a copy here: 

http://vncon.chronetal.co.uk/