Kerio free is considered a good firewall, but so is Outpost free. Outpost is almost always given the best rating for ease of use (understandable interface), it has lots of features, and it is extensible. People can add their own custom modules to it. For example there is an IP address blocker, supported on the same forums as the main Outpost program itself, whith regular updates of spyware/adware site lists posted for the users.
http://www.agnitum.com/
http://outpostfirewall.com/forum/index.php?
BTW, AVG includes a limited firewall in the latest version.