Take some Oreo
cookies. Carefully remove the top of each one, and put a small circle of wax paper in between the cookie and the filling. Watch as someone eats them.
If your roommate has posters of scantily clad women up, blow up a picture of their mother's face and tape it over the face on the poster. Very disturbing for the victim.
Put white 'art' paper like the kind used in pep rallies over the outside of a dorm room doorway. You and your friends get up a little early to stand out in the hall and clap and cheer when the roomate who's in on the prank jumps through the paper (like a football game). The next day, repeat the procedure, placing a candy or drink machine behind the paper. When the 2nd roommate (who's not in on it) tries t o burst through the paper for his/her applause, BOOM! S/He will hit the back of the machine and rebound back into the room.
Tape down the button on the receiver of the telephone and call them.
Fill two very large glasses full of water, tell a friend to place their hands on a table (palms down). Put the glass on top of one hand and ask if it hurts, when they say "no," place the other glass on top of his other hand and walk off.
Make the cook of the house mad and peel all the labels off the canned foods in the pantry.
Put baby powder inside a friend's hair dryer.
Glue a quarter to the sidewalk.
Many ideas listed here were gotten from Pranksta's Paradise