The Fitbit Ruby SDK by Zachery Moneypenny is a gem that allows developers to integrate the Fitbit API into their Ruby applications. This SDK has a passing build, but is not currently under active development.
Here is an interview with Scott Gimpel Founder and CEO of FantasyData.com. The site used to be known as NFLdata.com and provides NFL statistics. The site claims to have almost 25 million API calls /month during football season with over 300 developers working with it's API.