Where do I find a free (open source preferably) VNC management tool?

Remote Desktop Manager, I've found, is the simplest and most powerful remote connection manager. It keeps a hierarchical list of all connections, and can keep track of a huge number of types of connections: RDP, VNC, PCAnywhere, TeamViewer, LogMeIn, DameWare, FTP, SSH, Telnet, VMWare, Hyper-V, X11, Citrix, VirtualBox, etc.

The programs can store its list of connections in an MDB which can be put on a file share so multiple computers can have the same database, or the Enterprise version supports storing on SQL Server or Amazon S3.

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For the actual connection, I agree with Owen and say you should probably just use Remote Desktop. I've found it's the fastest and best engineered of the remote desktop solutions.

If you're trying to help another user on their logon (as you mentioned in a comment), you should install Remote Assistance on each of the computers. It's a feature built-in to Windows that allows screen sharing; I use it myself since I have to support a few people remotely, and it works very well. And yes, Remote Desktop Manager supports Remote Assistance connections.

If every computer is Windows - I wouldn't even bother with VNC. Just use the Remote Desktop Connection provided via Windows. (Note: Only included with Pro, Ultimate, and Business versions.) Couple resources to get you started:

http://windows.microsoft.com/en-us/windows-vista/Remote-Desktop-Connection-frequently-asked-questions

http://www.howtogeek.com/howto/windows-vista/turn-on-remote-desktop-in-windows-vista/