At work we have two levels of filtering, filtering at county level. It's good at picking up proxy bypass web sites but that's about it. So in school we have an RM provided solution called RM Smart Cache. Now it's good in some respects, but unfortunately has it's down falls.
Previously I used Censornet, this product does exactly what it says on the tin. It filters the Internet and it does it good! All users get presented with a log in box when they try to browse the Internet. This if your auditing trail of who has done what on the Internet. Now the old version used to be a single CD that was the install and it all worked straight out of the box. But with the new V4 it's a little bit more in depth. Which from my point of view is good. With the old version you couldn't do anything else with the box. But with the new version it's a basic install of Debian and then away you go.