Pour the contents of your roommate's change jar into a large bowl. Mix up a tasty load of flavored gelatin dessert in the bowl. Be honest: return the bowl to your roommate's room. Don't take a penny of the change.
If your roommate has several kinds of shower products that come in bottles, like shampoo, conditioner and body wash, switch around the contents of the bottles. This prank is relatively mild and reasonably safe to use even on roommates that you want to stay friends with.
Purchase a container of vanilla ice cream and an equal sized container of solid lard. Open the ice cream container and remove the contents, storing them in the freezer while you continue. Open the lard container and transfer most of the lard into the empty vanilla ice cream container. Use a long knife to peel away a thin layer of the vanilla ice cream. Cover the lard in the ice cream box with the layer of ice cream. Use a spoon or an ice cream scoop to dredge out one scoop of ice cream and lard mixture. Close the booby trapped box and place in the freezer. Hide all evidence of tampering, especially the empty lard container.
Obtain a highlight marker. Sometime when your roommate is not around write a few choice remarks on a suitable item of clothing. Under black light, like at a bar or club, the markings will glow. You may want to test this on some clothes you don't care about to find a color combo that hides the markings under normal light before moving into revenge mode.
Put your roommate's mail in the toilet. Just kidding, that's a federal offense. Seriously. Don't put your roommate's mail in the toilet and leave it there. Take it out and let it dry thoroughly. On a related note, use a cotton swab to wipe a drop of toilet water on the back of your roommate's stamps for a special taste treat.