Trepia™ is an interesting twist on chat clients.  Instead of having a buddy list of friends, Trepia™ searches for buddies based on proximity.  People in your buddy list won't be friends you already know, they will be the people logged in who are physically closest to you.

Here's how Trepia™ describes their technology:

Trepia™ performs a Progressive Proximity search for nearby people using known geographical information about the networks you use. It searches from the inside out, first adding people local to you, then branching out to others.

Sounds cool, right?  All I want to know is: how do they plan to make money?

(Thanks Steve for the pointer to the cool software.)