I've been Shaun from Shaun of the Dead, very easy costume to make. I've been Lil Jon in years past. I really want to be Stuntman Mike from Death Proof this year but I don't think I can make that happen this year.
Shanny and I made Haxmat costumes that were really cool and a big hit one year. We got a pair of those one piece white plastic/vinyl/whatever they are painters suits, they're like a few dollars each if I remember right. Black rubber boots and big black rubber gloves. We made Hazmat cutout logos with the radioactive symbol and spray painted it on the back of the suits. Then we got respirators goggles, and some little blinking devices. The coolest part was we got a plain white bucket and put nuclear hazard stickers on the out side, then we put a few glowsticks in the bottom, then dry ice and a little water so the bucket glowed and smoked. And finally we put all our beer for the night in the bucket. When we arrived at the party we came in gently carrying the bucket and clearing a spot in the middle of the room, no one recognized us with the masks on. We used the little blinky devices to "scan" the bucket and whatnot. Then we pealed the lid off, each pulled out a beer, and took our masks off. Everybody was like yay!