Type in a keyword or keywords and any time a Twitter is created that includes those keywords, you’ll be notified via IM or SMS.
Notifications are likely to also include being delivered by email and RSS. Twitter CEO Jack Dorsey told TechCrunch that the company eventually plans to add historical search, and a public API for keyword searching (both real-time and historical), as well as the ability to "define searches by friends, geography, time and/or language."
This opens up Twitter to become the ultimate buzz tracker, for those who are interested in what's being talked about at any given moment in time.
I've already witnessed industry gossip being exchanged via Twitter, including public bouts, as well as more serious "breaking news" related to the tech industry. However, as Twitter widens its userbase, through collaborations, such as the recent MTV partnership, marketeers and journalists will be able to mine the realtime micro-blogging conversations of the service's many users. Additionally, Twitter could make some of this "buzz" more public and better organized on the site's front-page. A tag cloud of recent and popular keywords would give a very raw but interesting snapshopt of what's "hot", and if an algorithm was developed to categorise certain buzz words into categories, Twitter could offer a buzz tracker for different broader topics e.g. Tech or Entertainment etc.
Think Techmeme meets Twitter.
Mashup Twitter's search API with other third party services, such as a music store or movie theater guide / review site -- as suggested by Dave Winer -- and buzz could translate into further actions / social activity. Now that would be cool.