The Web Accessibility Initiative (WAI) contains a wealth of info about web 2.0 accessibility for those with disabilities. My favorite feature is that it provides scenarios of both children and adults who have disabilities, the barrier that they face, and how to overcome that barrier.
In this article there are several common Web 2.0 applications discussed. There are also examples of common accessibility problems with web 2.0 pages. This best part of this web page is the list of ways that web designers and companies that have disabled employees can check for accessibility and make their pages accessible if they are not.