Google already knows too much about me, yet Google Latitude's Location History and Location Alerts features are still creepy. But they're useful, because together they learn your usual hangouts and know when you don't need notifications of who's nearby.
The Location History feature does just what you think it would by allowing you to view your previous Latitude locations. And the Location Alerts are little notifications when a friend is nearby and spare you from constantly checking Latitude. Since it'd be pretty annoying to get a notification that your boss is in fact in his office every time you go to work, you can use both features together and let Latitude learn your frequently visited places. It'll keep track of patterns and skip sending unneeded alerts. Creepy, yet oh-so-useful. [Google Mobile Blog]