Web 2.0

The best information on Web 2.0 is a two year old article from the people who coined the word, O'Reilly. A lot of good stuff in here about how the only companies that survived 1.0 already had 2.0 aspects going for them. Ahh yes, it also talks about how it puts good use to the long tail, so everyone's in.
Now for an example of a Web 2.0 company. Well, we all like YouTube, but how about something a little more educational like Wikipedia. Ok, for something really obscure: Twitter.