Twitter is a webservice that lets you check-in with what you are doing by text message, IM or over the web. Think of it as an away message answering machine. While people have been using their away message on IM clients as status updates for as long as I can remember, if you missed a friends away message it was gone forever. Twitter gives you a tumblelog of your friends incessent status updates that can be forwarded to your cell phone or read on the web.
It's the perfect tool for conferences like SXSW. Keep track of where your friends are or even ask questions (like where's the next open bar?).
They even set up a Twitter-screen at the conference where you can see the latest status updates of people who have added Twitter: SXSW as a friend.
Top photo by Adactio