The case of Wikipedia: I’m actually linking to a section of a larger article. The complete article is a little slow, but there’s a nice section here on the Wikipedia — the awesome online encyclopedia that’s often a first-stop reference for me.
Another secret of the success of Wikipedia is related to the incentives that contribute to a “creative construction” of information, rather than a “creative destruction” of it. As noted earlier, I expected Wikipedia to be engaged in an endless war among reliable contributions and graffiti attacks that would have blocked the development of the Web site. In reality, that has not happened, basically because all changes made to any article are stored; it is possible to undo any unapproved modification with a single click. This makes the activity of littering a page extremely more expensive for an individual (in terms of time and reputation), than it is for anyone else. Therefore, also in this circumstance, it is a matter of costs.