Developer tools company Fog Creek have produced a rather useful tool for those of us who have to occasionally fix other people's computers. A web-initiated secure VNC session, routed through Fog Creek's servers, Copilot allows you to work with someone's computer simply by making a request at a website. Windows only at the moment, but the GPLed code is available if you want to port it to other platforms!

Yes, there is a fee, but at $9.95 it's less than the cost of your time when trying to debug VNC or RDP connections through someone's firewall...

^*Yet Another Useful Tool - and this was an intern project too!