Google Latitude.

There are two primary ways to use Latitude right now:

1. On your mobile phone: visit google.com/latitude from your phone's mobile browser to download Google Maps for mobile with Latitude. We currently support most of the popular smartphone platforms: Android, Blackberry, Symbian S60, and Windows Mobile, and we are hoping to see Latitude on the iPhone soon. It will be available through Google Mobile App, and you'll just need to download or update the app from the App store to find Latitude in the Apps tab.
2. On your computer: go to google.com/latitude from your browser and add the Latitude gadget to your iGoogle homepage. What's neat is that if you've installed Google Gears or if you're using Google Chrome, you can choose to automatically share your location from your laptop or desktop computer — no smartphone required!

Sakset herfra: http://fatihsyuhud.com/2009/02/06/google-latitude/