It's called http://idonethis.com.
We email you on a daily basis asking you what you got done today. We put your email response into a calendar and check off the day. Look at your calendar to see your streak from yesterday to motivate you today.
We posted the site on HN back in January and got some great feedback which we incorporated (http://news.ycombinator.com/item?id=2064038) along with some nice press coverage (http://thenextweb.com/lifehacks/2011/01/03/idonethis-have-yo...). We're at a few hundred users, a good proportion of whom email us on a daily basis and tell us that that the site is helping their productivity, helping them quit smoking, reminding them to exercise & diet, etc.