October 2007 Microsoft launched this product , but I came across it recently through the MS download notification newsletter which a receive every saturday.
What is it ?
With HealthVault MS has created a platform for registration your medical data. You can decide to share this data with your doctor etc. This data can also be collected from all kind of devices like bloodpressure meters etc. by means of a HealthVault Desktop application.
The service is only offered in the US at the moment. I predict that this service will be very populair based on the success of products like the Nintendo Wii Fit.
Of course Google has a similar product, called GoogleHealth. I almost signed up untill I saw the User agreement.