Google have released Google Latitude, an update to their Mobile Maps app which adds the ability to share your location with other users, track friends' movements and trigger calls, emails and IM conversations with them directly from Latitude's user interface. Available for Android devices now, G1 users in the US will be receiving Maps v3.0 in a system update soon.
Since using GPS in this way can be a contentious issue, Google have given Latitude users the ability to restrict location information on a contact-by-contact basis; alternatively it will let you enter a false position manually. Results are shown either on a map or in a list, from which individual people can be contacted without having to exit Latitude and go into your phonebook. Right now, the app works in 27 countries; there's also an iGoogle plugin available for desktop use.
Since not everybody uses a T-Mobile G1, Latitude is also available for most Windows Mobile 5.0 and above devices, most Symbian S60 handsets, and most color BlackBerry smartphones. Versions for the iPhone, iPod touch and many Sony Ericsson handsets will apparently be available soon.