HTML 5 – as-yet unreleased, but shaping up well – contains a specification for finding the current location of the user. The API, if your browser supports it and you grant the web application access, returns your latitude, longitude, elevation, speed and some other details. (If your web-capable device doesn’t have GPS, these details will be estimated using your IP address and other factors.)
A couple of weeks ago, I created a page to test this feature. If your browser is geo capable, this will reveal exactly what data about your location is being sent to web applications that ask for it.
If you’re a developer, here’s how I created the page.