I've just completed my second contribution to Google's "Knol" - their run at Wikipedia's dominance in the free knowledge marketplace. Both Wikipedia and Knol materials are written by unpaid volunteers, but there are important differences.
Wikipedia has been around much longer and therefore has a far more extensive and detailed range of materials. Wikipedia contributors are anonymous and the style is collaborative - anyone in the world can take a shot at any Wikipedia article and edit anything they want into it. Fortunately, at least for important topics, Wikipedia has many active volunteers who generally correct false or misleading or partisan articles and edits pretty rapidly and fairly.
Knol requires writers to identify themselves and their affilliations at the head of their articles. They even verify the writer's identity by making an automated phone call to the writer's phone. They also encourage edits by anyone who is interested, but these edits must be approved by the original author before they appear. It will be interesting to see how Knol fares against Wikipedia. Right now, Knol is David vs the Wikipedia Goliath - but Google is a very successful company and, with them behind Knol, and able, as the premier web search engine, to make sure Knols are well-represented in the results when people do searches!
I am using my Knols as materials for my U Maryland students, replacing class materials previously only available on the U Maryland website.