Google has just released Latitude their update to Google Maps that allows you to know where your friends are.
You can get the software for Blackberry, S60 and Windows Mobile. Android is expected to launch in a few day and an iPhone version as part as Google Mobile App at some point.
To download just point the phone browser to google.com/latitude.
Once installed, sign in with your Google email, then add friends and a request will be sent to them.
It doesn’t track real-time, but sends the last logged location.
Google bought Dodgeball a while back which was a social media location based service but development seemed to die, though Latitude does seem to be the bastard child of Dodgeball.