On Fitocracy, tracking your workouts earns you points/levels, unlocks special achievements, and fulfills "quests", which are basically exercise milestones that aim for workout variety and progression. As you progress in your training, Fitocracy offers more quests, increases the level up requirements, etc. We also try to make the experience as community focused as possible, so the site actively encourages you to follow others with similar fitness approaches, comment on workouts, etc.
We have been in invite-only beta since mid-February and have become fairly popular on Reddit and SomethingAwful. I know there are plenty of folks on HN that are interested in fitness so I hope this will be of interest to some of you.
URL: http://www.fitocracy.com
You can sign up using this invite link: http://ftcy.co/jaBLMa
We're still working through a long list of bugs and features to get out, but do feel free to offer any feedback. Hope you all enjoy!
You can find me on Fitocracy at: http://www.fitocracy.com/profile/xenowang and my cofounder at http://www.fitocracy.com/profile/dicktalens (seriously, the way our usernames may sound to some is purely coincidental)
-Brian
Interesting side note: We started working on this in October 2010 and I had no real prior experience in front-end design at that point. I've basically been cutting my teeth in front-end design (design principles, Photoshop mockups, graphic assets, HTML, CSS) on Fitocracy.