I've just added Google Latitude to my phone. Latitude is an new addition to Google Maps which allows you to see where your friends are at any given time. Your phone's location is determined by your triangulated distance from phone masts (to give an approximate location) or your phone's GPS.
You can change you privacy setting so certain people, or no-one can see where you are, or you can limit it to just say what city you are in.
In theory it sounds quite cool - you can look at your map and see that mayeb your friends are nearby and might fancy meeting up for a drink. It would also be quite cool for companies that want to track field engineers but don't want to shell out for expensive fleet tracking software. You can integrate Latitude with your iGoogle page so see where all your "friends" (or employees) are on your computer.
On the other hand it is quite creepy. Always being tracked? It takes Facebook stalking to the next level. With this you literally could hunt someone down! Hmmm... dubious.
I have added it for now. I doubt I have enough techie/geeky friends to make much use of this!
W makes a good point - this is a bit like like the Marauder's map from Harry Potter.
/edit - now that I think about it - for all the paranoid parents out there this will be a god-sent!