Flash Mobs is about doing something entertaining uplifting in a normal place, surprising every random crowd that happens to be in that very spot.
Lately I've been watching a few of these Flash Mobs at YouTube, and decided to link to some of the ones I find best.
First 3 classics that kinda started it all (afaik):
Sound of Music: http://www.youtube.com/watch?v=bQLCZOG202k
T-Mobile dance: http://www.youtube.com/watch?v=6-3kkqXX85c
Frozen Grand Central: http://www.youtube.com/watch?v=jwMj3PJDxuo
T-Mobile welcomes home random people in the airport: http://www.youtube.com/watch?v=NB3NPNM4xgo
ImprovEverywhere makes some good ones too, like
Star Wars in the subway: http://www.youtube.com/watch?v=J5gCeWEGiQI
Ghostbusters at the library: http://www.youtube.com/watch?v=wKB7zfopiUA
Best game ever: http://www.youtube.com/watch?v=_Nbkbss7i5s
Jamie Oliver is also with it: http://www.youtube.com/watch?v=tDEJR-6paB0
And the more crazy (but still funny) ones: http://www.youtube.com/watch?v=FFgbSgUej4A
There is a ton of these videos at YouTube if you want more.