One of my pledge brothers went as Party boy from Jackass...had a zebra thong, the tear away pants, and the run/dance totally down. He shaved EVERYTHING!!!1
One girl went as a "one night stand" as a night stand

She was the table and had a lampshade as a hat on, with a box of condoms, cigarettes, bra, panties, and other things you would need for a one night stand all draped or glued to her table.
I went as a doctor...I went and got a fisher price doctors kit from toys 'r us, and gave check ups as the night went on...I even handed out prescriptions for more drinks when my diagnosis was that the patients weren't drunk enough. I also learned that some people are really super freaked by needles. I had a fake syringe that had a fairly realistic looking needle (it was like the tip of a mechanical pencil) and people flipped out on me! One girl ran away from me!

as a pre-med I don't get it.
Two ideas that didn't happen, but would have been great: Our party was with APhi, and one of our seniors basically got himself into a lot of trouble with them at one of our date parties (with my date no less) and so since he's no longer allowed to show his face there, he nearly went as an APhi! It probably would have been pretty ugly, so I'm glad he didn't but it would have been awesome.
Other idea was for a guy whose last name is Sheets. He thought about making shelves and putting bed sheets on them as his costume. He would have been sheets!!!