SquidGuard

anon writes: "Web filtering software, complete with a community of users collaborating to maintain blacklists. Used in prominent K-12 environments to satisfy CIPA requirements and to protect the delicate eyes of small children. For more, see Multnomah's summary, or the official squidGuard website." I can vouch for squidGuard; though I've not used it specifially for filtering (rather, for more complex ACLs than squid itself prefers), like squid, it is extremely useful and reliable.