I have tried several internet filtering services, such as BSafe Online, Christian-Net, and the Home & Family Internet Filter.
However, I have found K9 Web Protection to be just as good or better than these other filters... and it is completely free! If you have children who access the internet, or if you want to prevent temptation for yourself as I do, then I highly recommend giving this service a try. Not only does it allow you to set which categories you want to filter, it also keeps a log of all the sites that the computer's users have attempted to visit. These logs can be checked and reset and the settings can be changed only by the one who has the password.