Wikipedia, Wikibooks, and all of the other Wiki products available online are the epitomize the collaborative nature of Web 2.0. Together authors draft informative articles, books and definitions available to all who have an internet connection for free. Also, unlike discussion boards or other prominent communication tools on the web, wiki's promote cooperation and consensus. People are encouraged and to discuss and work out differences, eventually reaching a consensus.
There are some skeptics of wikipedia with regard to its accuracy and scholarly integrity. I have personally witnessed this skepticism when completing my undergrad degree a few years ago. When I cited wikipedia as a source of information in one of my papers, the professor cried foul, saying that it wasn't a legitimate source. The fact is that the accuracy of Wikipedia has been studied, and it has been found to be as accurate as other sources. In fact, in the article Studying Cooperation and Conflict between Authors with history flow Visualizations (2004) the authors determined that vulgar material and mass deletions of information are corrected in less than three minutes on average. I would also argue that wiki resources are better than traditional reference materials because they are flexible. If new information comes out, the wiki's are easily corrected, where an outdated encyclopedia lives forever (until someone throws it away).