Abracadabra is not just for Halloween. If you want to throw any kind of party or are a theater person, Abracadabra is the spot for your costume and prop needs (especially if you are interested in horror -- they have fantastic masks). The prices are better than some of the other costume shops around the Union Square area.
I love their wig section - wide variety and good quality. These wigs will last beyond one night.
They carry a large amount of kids costumes in addition to their adult section. It is a bit chaotic, so prepared to be overwhelmed by the volume and choice.