Healthy Lemur is an app that lets you publicly commit to exercise:
This is how it works:
1.Link your foursquare account
2. Select the number of days per week that you're going to exercise
3.Link your facebook and/or twitter accounts
And you're all set!
We track every time you go to the gym and every Monday, we will post on facebook and/or twitter account if you have reached your goal!
Kind of a poor man version of those withings scale that tweet your weight every week.
The app also tracks checkins on running tracks, swimming pools, etc.
The app also replies to your check ins at gyms telling you how many days away you're from your goal.
Each user also has a profile page that gets tweeted/posted when you reach (or don't reach their goal)