The essence of a Web 2.0 site is that the interesting content is provided by fellow users not by the owner or host. The owner's role is to provide the infrastructure and framework within which the site operates, to police the content and market the service (though this too can be done virally by the users). The Wikipedia reference uses many words that are pretty incomprehensible, so the concept may be best illustrated by examples. Wikipedia itself, of course, is a classic example.
In the more commercial world eBay would be another example: in this case buyers and sellers are connected by the auction site and buyers are validated by customer feedback. Although Amazon has a business of selling its own books and other products, it has used Web 2.0 in a big way: first, in allowing customers to post reviews of the products it sells; then, by offering sifting and sorting of its products based on most frequently purchased and customer rating; then, by offering its customers the option to sell their surplus printed books and other products; and, now, to allow authors to publish eBooks.
And then there are the many forums around areas of common interest. I offer a few examples that have helped me improve my web skills (PHP Freaks, Source Forge), but there is probably one or more on any subject you care to search for. Many of these use open source platforms like PhPBB and we can offer this to our clients as well.
You get the picture, I hope. It is this concept that excites me and so I would be very interested to discuss your ideas for any such development, large or small.