Mac OS X (10.4).
Now I hear some of you already shouting "HE WAS LOOKING FOR SOFTWARE YOU *******". Now, indeed he did, he never specified what his system hardware was, or the price, or what OS was involved. Mac OS X is also software, and comes with built in parental control that can accomplish exactly what is needed here. One can build an approved web-sites list, as well as ones that are blocked. Can stop certain programs from being run, and all kinds of rules that stop people from e-mailing certain people from Mail.app.
http://www.apple.com/macosx/features/family/