http://StickK.com is exactly that. You sign up, make commitments, have a friend verify, and then put your money where your mouth is.

If you have trouble meeting commitments, this website may be able to help. You choose what the commitment is (say, exercising five times a week for the next twelve weeks). You then add supporters/friends/referees to help verify that actually do these things. You then (optionally) send money to the website.

The idea is that you regularly check in and mark the commitments as completed when they are done. If you specified that a friend has to verify that you did actually do what you committed to do, then they need to go to the website and affirm as well. If the above doesn't take place (they do allow a small grace period), then whatever money you put into the system gets donated to the charity of your choice. The twist is that the charity can be one that you don't support (or that you hate). You can even have the money sent to a person that you hate (talk about motivation!).

And of course, if you (and your referee) mark everything completed on time, then your money is returned. If I get my money back, I plan to use it as "found" money, and spend it on something to reward myself with.

So, if you have trouble making commitments to your commitments, this sounds like a fun/tricky way to motivate yourself to get things done. And if you still fail, at least a charity (or enemy) benefits!

Disclaimer: I am not affiliated with StickK, nor do I gain any benefits if you use it. I just thought it might be a cool tip/trick to share.