PledgeBank is the latest cool online social tool, and I suspect that it will become quite popular in the near future. The principle is simple: You pledge to do something if a certain number of other people pledge to do the same. The system can be used for a wide variety of things. For example, right now we have semi-celebrity Brian Eno who's pledging to "get three friends to sign up to the Proportional Representation Groundswell but only if 100 other people will pledge to sign up at least three of their friends too." Enviros, always on the cutting-edge, are already using it with pledges ranging from "will replace all the light bulbs in my house with low energy / long life bulbs" and "will buy only Fairtrade Tea, Coffee and Chocolate bars during the month of July" to "will clean-up the banks of the River Taff between Western Ave and Cardiff Castle" and "will Sign up for green electricity".Once you sign up you can read and write comments on the pledge's page and you get email notifications about the status of the pledge (ie. enough people signed up before the deadline?).
Other interesting pledges I've found:
and the best one: