Typically some sort of recognizable monster or ghoul, this costume is an old stand-by for many, including children and lazy-asses. Since so many of these costumes rely on what appear to be relatively expensive masks, deciding what to be for Halloween gets easier and easier for these people every year. I mean, once you buy that $70 Dracula mask, odds are you're going to want to get some use out of it.
The inanimate object
Usually some of the more creative costumes one sees, these tend to be larger-than-life manifestations of any number of items that are currently in the popular conscience. Unfortunately, the popular conscience only deems a few items worthy of larger-than-life manifestation, and so at any city-wide Halloween event you end up with 73 guys dressed as iPods.
The Female Costume
Ninety percent of women between the ages of 16 and 32 wear this costume. The costume is a sexy woman. Often they will make claims like "I'm a cat," or "I'm a cop," or "I'm a nurse," but invariably these are lies. Cats and cops and nurses do not wear high heels, short skirts, and button-down shirts half to three-quarters unbuttoned. Neither do genies, pirates, or witches. To make this costume, all you have to do is wear the most revealing clothes you own and then throw on some accessory that belongs to another genre of costume. Then, after the party, have sex with someone.
People really stick it to the president when they wear a George W. Bush mask and... now get this... a NAZI UNIFORM! Oh man! You know that G.W. is just squirming in his chair when he thinks about how he's being compared to the most hated and despicable person of the twentieth century. Damn! I always make a point of high-fiving these people and congratulating them on such biting and poignant satire. These people tend not to be registered voters.
This costume is typically modeled after an obscure character or image from the literary and artistic canon. The costume wearer takes great pleasure in telling you what he is after you have unsuccessfully guessed several times (he always forces you to guess). These may be things like Dr. Pangloss from Candide, the Screaming guy from the painting The Scream, or a weeping bounty-hunter/assassin from some bullshit French movie. Stick close to these people; they will be drinking the most expensive liquor at the party.