I've written before about Google and their knol project. As a quick recap, the basic idea is that a person can create a page about a certain topic. Others can add to or edit the page. The author may choose to place Google ads on the page.
Sound a bit like Wikipedia? There are a few key differences. First, a knol has information about the author. Wikipedia allows complete and total anonymity. This allows you to judge the validity of the article based on who wrote it.
It appears there are different levels of editing allowed. An author can either keep all editing only to authors, or they can open it up to 'suggestions' (where people suggest changes, and the authors decide if they want to keep it), or they can just open it right up for anybody to edit.
The other difference is that Wikipedia has never placed ads on their site. Users contribute and Wikipedia is a non-profit organization.
I wrote a blog post some time ago on the lunar phases. I actually get quite a bit of traffic to my site from people doing a search on how to tell if the moon is waxing or waning. So, I took that blog post, turned it into a mini article, and created my first knol. You can see it here. And of course, it's wide open, so if you see any errors, feel free to correct.