I need an easy way to jump on someone elses machine to help them solve a problem. It needs to be really easy for them to install (preferably doesn't actually require an "install" but just running an exe?). It must punch through any firewalls automatically using a relay server or P2P (so Remote Desktop itself is no use to me).

I've found commercial products like MeetMeNow but they're really expensive. I want something that you can either buy a cheap pack of sessions or minutes, or preferably something free.

I'm not in the business of commerical support and would only use it once every couple of months perhaps.