I’ve just stumbled upon Scholarpedia, a MediaWiki-based encyclopedia. The key difference between it and Wikipedia is its focus on peer-review of content. Of course, this immediately means that it has substantially less content than Wikipedia but less is more, right? Contributors are nominated and voted in by the public based on their reputation in their area of expertise, most being notable academics.
At present Scholarpedia seems to have quite a narrow focus (most current articles are about various kinds of adaptive systems in computer science). It will be interesting to see how the project progresses. Perhaps the most promising aspect is the quality of authors who have apparently signed up to write various sections. For example, if you could ask anybody to explain Genetic Algorithms, it would probably be John Holland. And who better to write about Hopfield Networks than John Hopfield himself?