Open Source URL Filtering Software?

hated asks: "I work for the government, and because of our stringent policy on not allowing 'personal' use from work computers we have been given a requirement from management to restrict certain types of Web pages...mainly porn. Now I am opposed to censor-ware as the next guy, but I don't make policy, just figure out how to implement it. I would really rather not use commercial URL filtering because of the price and because of the secretive blocked lists. Is there any sort of module that would work with squid? I am looking for a proxy based tools as opposed to host based...obviously far easier to implement. I hope /. readers can provide some insight."