UltraVNC

I really endorse this one. (Sorry that I missed it when I raised a similar request yesterday - which provides some fuller specifications.)

A Viewer log file would be pretty handy to look back on when billing clients for remote support you have provided (especially if they initiate via Single Click).

So it would be great if the Viewer had an option to write key session info to a log file:
(Session count/ID?), Date, Server IP Address ...