Behold my work’s site bans:
xkcd.com, blocked

pennyarcade.com, allowed

xkcd.com consists of crudely drawn (usually) figures making math jokes whereas Penny Arcade features a character named Fruit Fucker 2000 and an alcoholic DivX player. A year ago, there was a story arc involving the main character removing his genitals so they could rubbed against a 103-inch television.  I can see why they made the decision they did.