Underwear pranks are one of the most common and often used pranks.  Stealing someone's underwear is the very popular.  Since going into a freshmen's unlocked room is likely to ruffle some feathers, the load left in the dryer is a good target.  One of the most popular things to do is to put someone's underwear in a tree in a very public area.  Putting them into a bucket of water first makes them easier to throw into the tree.  The more pairs and the wider the spread, the better!  The other popular thing to do is to put them in a large bucket filled with water and find a large freezer to put the bucket in.  Once frozen, the large block can then be removed from the bucket and put on display in a public place.