Today there are a number of wearable fitness monitoring products with the Fitbit family among the more popular choices. While Fitbit has made an API available for developers to interact with Fitbit data in their own applications, products and services, the product is not open source.
There is an open source alternative API developed by Kyle Machulis known as libfitbit,
libfitbit is an implementation of the data retrieval protocol for the fitbit health tracking device. It also implements a synchronization client for synchronizing data with the fitbit website on platforms not supported by Fitbit currently.
The main goal of the library is to augment the services already provided by the fitbit website, allowing for developers to make interesting new applications that access the fitbit data in smaller time chunks on the local machine, as well as giving users the ability to back up their own data without having to rely on the web service.
Currently, the library only implements a proof of concept of the protocol and web interface in python. This will be turned into a C library, which can then be SWIG’d or otherwise interfaced to other languages.
(Fitbit hackers can find some interesting information on libfitbit’s reverse engineering page.)