I was browsing YouTube when I came across a few videos of Flash Mobs. These are really awesome. Here are some of the ones I like. Watch them, they're really cool. Theres also a video of one here in the Philippines at MOA.
Don't Rain on My Parade | South Street Seaport
Sound of Music | Central Station Antwerp (Belgium)
Beyonce 100 Single Ladies Flash-Dance Piccadilly Circus, London for Trident Unwrapped