blog entry, Google Latitude is useful for keeping track of your friends locations, and is making awesome improvements.
I've personally used Latitude to keep track of co-workers on trips to California and is indeed a useful tool. If you haven't checked it out yet, read the blog post and get cracking.
On a personal note one of the folks working on Latitude, and the author of the above blog post, I have known since I started working at Google almost 6 years ago. Joe worked in a different office, but we shared the same managers, worked beside each other in the same datacenters from time to time, and even travelled to Asia together for work. It is awesome to see how his career has grown... going from a hardware tech to writing code for a very useful public service. Go Joe!