http://www.uvnc.com/products/uvnc-sc.html

This is what I used when I built a remote support system for my employer a few years ago (I guess more than a few?). Since we offered flat fee remote sessions I simply had the download initiated after PayPal cleared the payment as part of the ticketing system. We replaced it with Go2Assist because of UAC issues, although I believe that has been since fixed. http://www.chunkvnc.com is also a good option from what I've heard, although it requires a repeater (included).

I only use TeamViewer when there is already a session on the machine and I have to assist the technician (which is a fairly constant thing since my boss has a somewhat unrealistic view of his own abilities.)

I like TeamViewer in terms of capabilities, but it lacks any method of automatically connecting with a single click.