Comodo's pretty good. It can be a little intrusive at 1st with all the questions, but it will settle down.
It does seem to get great ratings from security sites.
Another good free firewall that I put on my parent's pc is PC Tool's firewall. http://www.pctools.com/firewall/
They aren't tech savvy at all, and they were able to figure it out. Seems to run pretty light. They have an old P4 2.6 system with a gig of ram. It's a rule based firewall, so you can create your own rules if you like, but runs pretty good with the basic settings. I tried to access their computer and it caught me. Not that I ever tried to hack anything....