Google also provides opt-in location tracking via Google+ [0]. You specify who can view your location, and whether it is fine (GPS coordinates) or course (city-level).
My wife and I use this service and find it quite useful - it seems like a much more sane way to handle location sharing than to simply attach your exact location to every message you send by default.
Sadly, since it's buried in Google+, most people are probably unaware of it.
[0] https://support.google.com/plus/answer/2998354?hl=en
