e2guardian is a web content filter and a fork of DansGuardian. 
It filters the actual content of pages based on many methods 
including phrase matching, PICS filtering and URL filtering. 
It does not purely filter based on a banned list of sites 
like lesser totally commercial filters. e2guardian is designed 
to be completely flexible and allows you to tailor the 
filtering to your exact needs.

This requires a proxy server so squid is a recommended
optional dependency.