I know it's not free, but you can host the software on a linux box (even a cheap VPS will do nicely) and their pricing is *significantly* better than team viewer/etc, especially in bulk,. I also know they offer heavily discounted non-profit pricing.
It's mainly designed for the "run it for a few minutes, solve the issue and automatic uninstall when done" model.
The problem with join.me/teamviewer, is that the person has to read you numbers off the screen before you connect.
With Screen connect, they just run the app and you are automatically connected to the support rep.
I really like the way their software works because you can setup a "one click" installer, send them a webpage to pick a queue or make a separate queue for each rep. It works great between restarts and with UAT, as well has the option of letting you securely store the users password (so you don't have to keep asking them each time and the user doesn't have to tell you what it is either).