I'm very stoked about the new Track feature in Twitter. Essentially, it's like a Google Alert, or other systems where you get a notice when keywords you request or mentioned. Except it's real-time and its from all Twitters.
I was using this in beta form during the Techcrunch40 conference. I tracked "techcrunch40" and "tc40" and got all the updates that mentioned the conference directlymany from people attending, some from people just commenting on the goings-on. I really enjoyed getting this real-time commentary, especially while I was there.
It has other interesting uses, as well. For example, you can track @your_username to get public @replies from people you don't follow. Update: This usage may be slightly broken at the moment...
Even more interesting is the potential for new behaviors and applications to emerge out of this. I don't know what those are yet, which is part of the fun. But you can imagine services or communities establishing keywords to mean certain things, kinda like craigslist.