Back in the day, we used to leave a small file named
.plan in our Unix home directories, which others could read over port 79 with the finger command. It was a useful for quick status updates, to find out what people were working on (or where they were off to) without pestering them. John Carmack popularized it as a proto-blog, and the IM phenomenon brought a similar function mainstream via presence notification with custom status messages. But I've been frustrated at how difficult it is to log and broadcast a group's or even my own status messages.
Enter Twitter, basically an implementation of finger using the contemporary infrastructure of HTML and RSS over HTTP, with IM and SMS tie-ins. Now I have a .plan again. Maybe someday Ambient Devices will build me an LED sign to display it in a more physical manner.