Mikogo is an easy-to-use cross-platform desktop sharing tool

http://www.mikogo.com/
I have installed but yet to try. Install was painless but had to stop the auto start request.

Mikogo needs to be installed to start a session but joining a session only requires clicking "join" at the top of the Mikogo site.

They emailed me relentlessly for weeks trying to get me to use it. Ammyy is the way to go.