Last Weekend I ran the San Francisco ING Bay to Breakers which is as much street theater as it is a race. Elite runners line up first followed by seeded runners followed by people who actually intend to run the race followed by a lot of crazy people in costumes. There are also quite a few naked people in this race although they run the risk of being cited for undecent exposure.
The costumes are a lot of fun and I have to admire the people who put them on and then cover the 12K (~7.5 miles) to the finish line.
One of the funniest things of all, though is the San Francisco Cacophony Club who dress in Salmon costumes and start part way along the course, running upstream, against the tide of tens of thousands of runners. One year, though, they met their match.