Using the phones' built-in GPS satellite technology, Boost Loopt service could alert users whenever their friends are within a half-mile to 25 miles. The real-time tracking would only occur for those who have agreed to be located and had given the user their cell phone numbers.

At any given time, a person can temporarily cut out from being "spotted" by their friends as well.

Consenting buddies would show up as dots on a map with accompanying options to text-message or call the person, or send them photos. Participants could update their own location status as well, informing others with notes that they are, say, "getting lunch with Sam" or "in class," similar to how people often communicate online through instant messaging or other social-networking sites.

(description from textually.org)

You have to download a (java?) app to be able to use it.