I've signed up at Twitter. Their mission statement is pretty much to answer the question "What are you doing?". In many ways they are the status message for the web, and are comparable to IM away messages. You can see my current status at twitter.com/kurtmckee.
I'm not content to leave it at that, though, in part because I don't like the idea of hosting my data off-site, and also because I'd like to be able to integrate the information tightly into my website. Thus, I expect I will eventually create my own software to handle my status messages.
Leave a comment with a link if you sign up and try out Twitter!