Google Latitude + Google Location History = very cool and kinda scary
Before you get your panties and/or beard in a twist, you can delete your location history at any time. It's private, too: neither your friends or the public can see your location history.
Now, with that out of the way, are you grinning like a fool at the prospect of how awesome such a tool is, or petrified of the potential privacy issues? The paranoid amongst us will point out that government agencies could request your location history -- but at the same time, if you're innocent, you have a rock-solid alibi!
Then there are the value-added features: not only is it kinda cool to see where you've been, but you can also revisit your travels via Google Earth and Maps. You can also check at a glance where you were on a given day, which can be handy if you're one of those busy people whose days seem to all merge into one.
Finally, with Location History bonding to Latitude (it's a Latitude app, in fact), you can have Google send you a notification when you are physically near to a friend. Because Location History knows your regular locations, you can set it so that notifications only trigger when you are in an unusual place... such as a large park... at night... in the bushes.