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.
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.
What do you mean by "centralized manager"? Do you just want a listing of all computers and be able to double-click one to connect? or are you looking to see who is connected to what computer in real-time, disable remote connections, etc.? – Stephen Jennings – 2010-05-28T16:12:52.120
Having the ability to see everything in real time sure would be nice, but it needs to have the ability to just see all the computers and connect to them. – user23150 – 2010-05-28T16:19:37.720