SWACGirl has a lovely roundup of this phenomenon that's becoming more and more popular, which is what sparks this brief post:
I just want to say that I love this! Ever since ImprovEverywhere started doing their fun improvisational skits, I've been enjoying seeing the creative and surprising ways that these groups bring smiles to people. I really hope that this trend keeps up, especially with the simple funny things as well as the "random acts of culture."
YouTube, of course, has several videos listed under random acts of culture, or you can check out the search for flash mob. Google, of course has extensive results here and here, detailing just what these phenomena are as well as links to webpages and videos.
So, go check them out; I think you'll be delightfully surprised by the creativity and joy that this new fad brings!