You may not like my answer....

I've been asked a few times by both clients and students what I thought provided the best Parental Control & Internet Filtering service. They were often surprised by my answer. A parent. Depending on a program to protect your child from questionable online content is like using a piece of styrofoam to keep a dog in a room. It's only going to work for a few minutes until the dog figures out the secret and anyone on the other side of the barrier is just going to push it aside without much trouble. Filtering doesn't work because it suffers from many false positives, can easily be defeated by your child and web site owners. There are only two realistic choices you can make. One, only allow your child to use the Internet where you can visibly supervise their activity. Two, come to terms with the fact that like every other source of questionable material they can encounter (TV, movies, magazines, books, music.....), all the controls you can dream of placing on them mean nothing when they go somewhere else. The best you can do is offer them guidance.

Update (25 August 2006): Another reason to shy away from formal filtering services is the new political practises of owners of these services. They typically don't take very kindly to any kind of criticism from other web sites, and have been known to respond by blocking those sites by labelling them as porn or any other type of material that is blocked. Any organization that displays that kind of immaturity when they are critiqued do not deserve your business.