What is the best internet filter software that DOESN'T overblock?

I am supposed to look for a pornography-blocking program for all of our home's computers. I have had some experience with these in the past (my school had one) and all i know is: I hated it. It blocked many harmless sites, which was very frustrating.

So, when looking for a filter, I want to make sure it won't overblock, and I want to make sure that it won't slow my computer down while running in the background. If the filter does its job and just blocks pornography, I don't care about it, but if it slows my computer down and blocks sites that I know are harmless, I will hate it.

oh, and by the way, I know that many of these programs have features that allow someone to look at where other users have been on the internet, I neither need or want those features.

please help me, thanks
AJ