This is a fairly recent entry into the participatory web. It's a difficult tool to describe, and, when done well, still leaves people with the response, "Why would I ever want to do that?" Twitter is very much like a chat room, but without walls. It's like a chat field, where people speak their minds (limited to only 140 characters per statement) and those who have chosen to pay attention to that person, receive those messages. The key to using Twitter for a personal learning network is to follow people who work in fields related to yours and who twitter about their daily (or hourly) experiences.

• Twitter
• Various Twitter Clients