I've been playing with Trepia a bit. The Trepia software is used as an imprompto instant messaging system based on people near your real physical location.

The Trepia software works with wifi to send out a signal that other Trepia users can pick up. If two Trepia users are near each other, a message will pop up telling them that someone is close by. They can then exchange IMs back and forth. I think it will be fun to try this at a larger geek gathering where there are several wifi users willing to play around.

There's only a Windows client at this time. I asked where the Mac & linux clients were and the CEO told me they were coming in the future once the Windows version takes off.

Cool stuff if you ask me.