Fitbit is a new startup that developed a device to keep track of your lifestyle. i.e. Calories you burn each day walking the dog, jogging, sitting infront of the computer blogging, etc. This is what the fitbit looks like. 

Its small enough that you can wear it around the entire day. and at the end of the day, upload that information to their website @ fitbit.com and get all that information summarize. You can also compare your daily stats with your friends (yes its a social network), to get some sort of friendly & healthy competition going! Fitbit retails for about $99, which is really kinda affordable. The web service is free with any fitbit purchase but i think they are planning to charge for a premium subscription model.
Here is a video of how the fitbit technology works, and a Q&A session with founder James Park.