I've been working on a seminar project that addresses the idea of motivation, especially web sites or applications that seek to motivate you to do something. There are a few sites that I refer back to a lot, because I think they do a good job of it.

RunLog
At its simplest, RunLog is a place to track how much you run. But it is interesting in that it aims to use the unique aspects of social networking to motivate its members, as they describe in their about section:

"… RunLog was built to motivate people who spend most of their day working in front of a computer. Many people start running, to get in shape as their new year's resolution, and hopefully RunLog will help you maintain your goals through the support of an online social network."

43 Things
I'm not sure where the number 43 came from (perhaps GTD?), but 43 Things is a place to write down your goals, and then mark down your progress as you complete them. It connects people who have the same goals, and allows you to cheer people on or ask questions. Its a neat idea, but I find that the ability to write your own goals means that there are a zillion different versions of "eat healthy" on the site, but because of different wordings or spellings they don't show up as a match to anyone else's version (a problem with tagging systems in general). They have also diluted the simplicity of the site by expanding into listing places, as well as things you have purchased, and I think it makes the site less effective, and I am less motivated to use it.

People have known for years that making a list of goals is the best way to achieve them. But most of us never get around to making a list. 43 Things is great for that! Make a list on 43 Things and see what changes happen in your life. Best of all it’s a way of connecting with other enthusiasts interested in everything from watching a space shuttle launch to grow my own vegetables. So the next time someone asks you, “what do you do?” you can answer with confidence, “I am doing 43 things!”.