Firefox has good blocking mechanisms. Safari has PithHelmet. With both, you can set filters that will ban a porn site that is identifiable. And therein lies a problem. There is no absolutely secure method of blocking web sites. The answer lies in education, rules, and personal decision making - but you probably know that already.
Understand that porn sites will almost invariably set a cookie that allows them in. Set strict controls over cookies in whatever browser you use. That will not stop porn sites, but it will slow them down. Block popup (popunder) windows, porn sites use them repeatedly.
I cannot comment on Parental Control except that it cannot be turned on for an Admin user or network account (Tiger OS).