I've started using Plazes after some urging from Sean. I like the concept, seems like it could be much cooler than some of the other sites. Especially if people are using it at a conference or something. Then you could see other people who were physically around you.
The one problem I have? The Linux client kinda sucks. I noticed someone wrote an OSX client, so I looked to see how he figured it out and they sent him an API spec. I've requested the spec. I want to try and write a gnome-panel applet for this. I think that would be real neat. This could also be a real fun project for me. Both get me involved in Gnome more and to get my hacking skills up.