Y'all are weak:
Fill a garbage bag with VERY foamy shaving cream (no gel, foamy stuff), slide the opening end under someone's door while they're in there. Before the person has a chance to react, STOP on the bag. You'll flood their room with a cloud of shaving cream. 🙂
Place a seat of saran wrap across the toilet, under the seat though. When someone takes a piss without putting the seat up, they'll get a nice surprise. Even better...if they take a number 2....oh wow.
Take a small (dorm room sized) trash can and fill it half way with water. Lean the garbage can against someone's door, knock, and run away. When they open their door, the person and his room will be flooded. 🙂 (Make sure you don't fill the garbage can too far, when you tilt it, the water comes towards the top)
The king: My brother and his wife were telling me about the best joke. When they were in college, some friends got together while my bro's wife and her roommate were away for the day. They got inside of the room (I don't remember how) and moved ALL of the furniture around and made the room a mirror image of it's self. The two girls came back to their room and were completely dumbfounded. They left their room like that because everything was exactly the same, just on the other side. 🙂
Get a group of people together, and a large stack of newspapers. Find a room where someone has left their door unlocked. Crumple up the newspapers loosely and fill the room. Then lock the door. When the person finally finds a way into his room, he's welcomed by a room completely filled (to the ceiling) with newspapers. 🙂