From the Tools I use File, BrightKite

Would'nt you like to know if someone you have friend'ed on your favorite social network was in your neighborhood? Say to hook up with like minded sports fanatics, or nerds? There is just such a service and it is called BrightKite, a "Location Based Social Network" and it is in invite only beta. If you would like a BrightKite invite reply to this post and we will make sure you get one. With BrightKite you can post your location to predetermined place marks, or send your location via sms. Once you login to a placemark you can email in photos of your location that others who are on your friends list can check out the seats you had at the game, or the sushi you are about to eat for dinner.

And it works great on my iPhone.

Got BrightKite?