a Fitbit is a turbocharged pedometer that can measure not only how many steps you cover in a day, but how many stairs you climb (elevation), calories burned, distance traveled, etc... It's a tool many SP members use to help them be a bit more active through the day.
You can read more about it here. www.dailyspark.com/blog.asp?post=4_reasons
Check out some of the posts in the fitness section too.