I microblog regularly at identi.ca
If you don't know it, Identi.ca is a
microblogging service. Users post short (140 character) notices which are broadcast to their friends and fans using the Web, RSS, or instant messages.
If you'd like to try it out, first register a new account. Then, on the public timeline, enter your message into the textbox at the top of the page, and click "Send". It will go out on the public timeline and to anyone who is subscribed to your notices (probably nobody, at first).
To subscribe to other people's notifications, go to their profile page and click the "subscribe" button. They'll get a notice that you're now subscribed to their notifications, and, who knows?, they might subscribe back.