Dave's given his podcatcher a Twitter account. So, if you're interested, you can subscribe to notices, via Twitter, of what Dave's downloading.
"So what?" I hear you ask. You may not care what Dave's listening to, but chances are, someone does. Moreover, Dave's using Twitter as a messaging endpoint in what Rohit Khare calls a "syndication oriented architecture," or SynOA. Jon Udell and Rohit talked about this on IT Conversations a few weeks ago.
I'm using Twitter in a similar way in my class this semester. My students are writing servers that send updates to a Twitter account via the Twitter API. Anyone can then subscribe to those updates through RSS, via SMS, or simply by going to the Web page. Easy, simple, and pretty effective.