I use
Logmein.com at work, the free version basicly just a remote desktop management suite allowqing you to add and sort an unlimited amount of clients.
Create a link to download the logmein installer with your accounts details already in it and pop it on your website.
Then any customer can just click the link and bingo you have a connection.
Not bad for free. thou it is not as fast as RDP or TeamViewer.
Anybody else use it?