External cycle computer sensors

[marknotgeorge]

I've been making progress with my Windows Phone cycle computer app. So far, I've managed to import a route from a GPX file in the SkyDrive (cloud storage), and display it on a map control on screen with a nice elevation graph.

So far, so good. Now I need to start thinking more about the app's internal database, where rides and routes will be stored. Although I'm not yet ready to actually code for external sensors, I need to think about leaving space in the database.

So what external sensors are useful to cyclists, and what information do they send?

As far as I've been able to work out so far, there's:
Heart rate - simply a beats per minute number?
Speed - is this wheel rotations or bike speed?
Cadence - simply pedal rotation speed?

Any more?

Thanks muchly...