That's what I like about dansguardian (www.dansguardian.org), it's a true content filter;
while you can use url blacklists with it, it goes one step futher and actually analyses the text content of the page with certain words worth negative values and other positives and if the total value of the page is past a certain configurable threshold, the page is blocked. For example, "breast" might be worth 10, and "cancer" might be worth -10, so a page about "breast cancer" will pass but not a page about "breast & sex"... so using a proxy site will not foil the filter.

It works actually quite well. I installed it at the begining of the school year (I admin for a private highschool), and I've had only 2-3 false positive pages that I've had to whitelist since.

Ammo