Awhile back, HN users rguzman, peng, and I (met on HN and) built http://idonethis.com based on the Seinfeld productivity secret and launched it on HN. We got some great feedback and kept moving forward with it.

We recently crossed 200,000 things done (http://thenextweb.com/apps/2011/09/22/idonethis-announces-it...) and a bunch of people have told us that it's helped them tremendously.