Heh, even though I haven't been in college for a while, I found 380 Ways to Freak Out Your Roommate pretty funny.
Some of my favorites from the list:
"Buy a copy of Frankie Yankovic's "Pennsylvania Polka," and play it at least 6 hours a day. If your roommate complains, explain that it's an assignment for your primitive cultures class."
"Create an imaginary cat for a pet. Talk to it every night, act like you're holding it, keep a litter box under your desk. After two weeks, say that your cat is missing. Put up signs in your dorm, blame your roommate."
"Eat a bag of marshmallows before you go to bed. The next day, spray three bottles of whip cream all over your floor. Say you got sick."
And this one just might be my favorite:
"Make pancakes every morning, but don't eat them. Draw faces on them, and toss them in the closet. Watch them for several hours each day. Complain to your roommate that your "pancake farm" isn't evolving into a self-sufficient community. Confide in your roommate that you think the king of the pancakes has been taking bribes."