DansGuardian/IPCop

It's a Open Source Webfilter. Free for non-commercial use and thus it is widely spread in universities, schools and libraries. It works as a Proxy with URL and keyword filtering (and also with the PICS-Standard). It's often used on a IPCop machine, however, the author from DansGuardian doesn't like it.

More Info: http://dansguardian.org/