Now seems to be a good time to mention it again, firstly because I've got into Twitter a bit more and it's one of those things that you'll gradually begin to hear more people talking about (the FT even mentioned Twitter on its front page). Secondly because Tom's Tube Twitter feed is working again after TfL revamped their website.
In a nutshell, Twitter lets you tell everyone what you're up to in 140 characters, from either the web, IM or your mobile phone. You can also "follow" people or add friends and catch up with their various goings on (good way of finding out what people are having for breakfast if you're that way inclined).
Thanks to Tom you can stalk your Tube line. He said: "Every fifteen minutes, my server will check the Tube status and send any delays to the relevant Twitter accounts. You can then quite easily subscribe to individual Tube lines and get updates via whatever means you deem appropriate as you would as a Twitter user.
A neat side effect of this is that there is now Atom and RSS feeds for each Tube line which you can subscribe to and get these updates sent directly to your RSS reader too. Twitter can also serve as a historical home for Tube problems from now on."
BTW my twitter account is here. As I'm not (quite) addicted to it yet, it's mostly updates on when I go to pilates, what I'm having for dinner/lunch, but also will give you a tweet with any other random thought I have.