I will if you will - via BBC News

PledgeBank is a site which my colleagues and I have built which lets you tell other people that you will do something, but only if a certain number of other people will promise to do the same.

For example "I'll set up a residents' association for my street, but only if four other people will come to my house to talk about it." People then sign up, and the pledge either succeeds, or it times out and expires.

Another great idea. You can sign up to see if there's any pledges local to you as well. - Clicky

[EDIT: I've signed my first pledge, which is to give 1% of my earnings to charity - details can be found here]