There are programs like NetNanny, but the problem with NetNanny is that the adult site needs to be registered with NetNanny in order for it to be effective.
Otherwise, if the computers are on a shared internet contection with with a PC doing the routing, you can get a program called Win Proxy. It a software router program that offer the following :-
* Website / Keyword Black Listing
* Adult Site Blocking
* Job Serch Engine Blocking
Otherwise, I do not that if you have a Hardware Router. Eg. My Netgear DG-814, you can blacklist website and keywords.
Just for the $#!+s and giggles value of it, I blacklisted the word "Transformers" and "Dragon Ball". My flatmate was plenty pissed when he found out what I did.