Google latitude the new and very cool feature from Google in association with Google Maps. Simply download the latest version of Google maps and from the right hand menu select Latitude.
You are asked to sign into your google email account; from which point you can add friends as easy as searching for there email addresses. It's that easy to setup!
Once connected to your friends, although not 100% accurate you can see your friends locations...the best i've seen so far using GPS is a distance of 40metres.
No more sneaking off to the pub though after work guys if your wife has a smart phone eh...
Download and More Info here:Google Latitude