I'll give some good ones though i saw last year here at pitt.
Myspace
-Somebody actually went out with this big cardboard thing over them that thy had done up like a myspace page and their head came out where the picture would be.  They also had an mp3 player with speakers on their back looping a song over and over.  It was a sick costume.
Trojan Condom
-This is my buddie Tyler. Nuff said.  Wrapped condom.  This has been on college humor.  Some of you might have seen this before.
Anchorman News Team
-Fully retro Ron Burgandy and the rest of the crew.  It was pretty legit.
Last year my friends and I went as the cast from Top Gun.  Maverick, Ice Man, Goose, and Viper.  We went to the Army/Navy Surplus, and went all out.  Fully flight jump suit, actual replica patches, the whole works.  Here's a picture:
Feel free to copy those, kinda played out.  But like I said, I'd tell you mine, it's just that it's too original and hilarious this year.