We've had dress up stuff a few times, mostly at the behest of our dm. At our Halloween game this year, we got bonus eeps for coming in costume. Bonus bonus eeps for dressing as our characters. Only myself and one other did that. There's another person in the group who was playing a seshayen (from d20 modern. it's one of our allowed player races for this dms game) and we thought it'd be awsome if he dressed up. We thought some blue face paint, fake wings, and a few extra pairs of googly eyes would be extra cool. He thought it was beneath his dignity.
When our gaming session fell on Talk Like a Pirate day, I wore my pirate costume to the game.
The dm has a fabulous broad brimmed pink leaopard print at, known as the pimp hat, that comes out for props occasionally. It's best remembered use is when the dm was a player in my game, playing a faen sprite, and the hat was his hat of disguise.
Best of all though was when we were playing a short mutants and masterminds adventure that tied in with our D&D plot. The dm took one of tose nylon book covers and put it on his head with a domino mask. It fits just like a superhero costume, and the corners stick up to make little ears. Priceless.