Have you ever wanted to do something, but backed down because your friends or family wouldn't join you? I can't even count the number of great concerts I missed out on because I couldn't find anyone to go with.
Pledgebank aims to solve this problem--and i'm not just talking about increasing the number of concert goers. You pledge that you'll do X given that Y people also do X. Most of the pledges are for noble causes--blood donation, purchasing a hybrid vehicle, donating money to a charity in need. I'm not sure if this will work the way as planned, but it has the potential to be something great.
