A lot of sites have been getting press recently as attempted rivals for Wikipedia, attempting correct one or another of its perceived flaws (that it’s too amateurish, unreliable, biased, etc.), or at least get in on the staggering amount of page views it receives. Below is my roundup of the ones I know about…
Citizendium - the most well-known to Wikipedia at the moment, I think. It’s run by Larry Sanger, Wikipedia’s less-famous co-founder, and it’s meant to be a more civil and better-educated version of Wikipedia, but with the same technology and the same multi-language support. To edit the site, you have to be registered with your real name, and regular users are meant to give deference to those who can prove that they have expertise on a particular subject.
Scholarpedia - it bills itself as a competitor to Wikipedia, though it’s not really. In Scholarpedia every article is written by experts and professionals, which means that only a small number of articles can get written, and presumably many topics can’t be addressed at all (who, after all, is an expert on, say, “Wheel of Fortune”?) Though you could make a credible argument that such topics don’t belong in an encyclopedia in the first place. Currently the articles that do exist are mostly on scientific topics. It does offer an interesting niche, of articles that are credible enough to be cited directly as references (I don’t know if anyone out there is citing Wikipedia - it seems somehow both implausible and inevitable.)
Knol (no link yet, but you can read more about it on Wikipedia, including a mockup page - Google’s planned answer to Wikipedia, to some extent. Like Citizendium, everyone will have to use their real name, and if you want to start an article you have to submit your picture as well. Once you start an article, you’ll basically “own” it, though others can edit it. Thus, the focus will be more on individual pages than on trying to create some sort of unified site. My guess is that this particular site was at least partly inspired by Wikipedia’s uncanny ability to get to the top of Google search results on just about any search term; that’s somewhat confirmed by the Google executive in charge of the project stating that each page on their site “is meant to be the first thing someone who searches for this topic for the first time will want to read.”
Freebase - an encylopedia run by Metaweb, using their proprietary semantic wiki technology; which means that, as far as I know, it’s the first and only semantic encyclopedia, i.e. the only encyclopedia whose data can be queried and aggregated. The interface is really geared toward its database-like style, with lots of field names everywhere for users to fill in. That makes it somewhat hard to read for general reference, and I don’t know if anyone uses it for that purpose. But as a proof-of-concept it’s neat, and for what it does it might be the only game in town, at least until Wikipedia itself bothers to add semantic capabilities…
Conservapedia - an attempt to counter a supposed left-wing bias on Wikipedia, by offering a version with a stated conservative and pro-American bias. Will they succeed? Who knows. They claim to have 20,000 articles, which is quite impressive given that they’ve only been around for about a year (since November 2006). That’s already comparable to the size of, say, the Greek-language Wikipedia. Who knows? Maybe the concept of an opinionated wiki has potential.
By the way, I don’t know if there’s any comparable site on the left - I know about dKosopedia, but they aim to be only a “political encyclopedia”, as opposed to a general-interest one.
Lohipedia - no, it’s not a site about Lindsey Lohan but rather an encyclopedia focused on user ratings and “karma”. The more other users trust you, the more permissions you have - though it appears that anyone who visits the site can still edit anything. This one uses the application Lohimedia, which runs on Ruby on Rails (MediaWiki, by contrast, which powers Wikipedia and some of the rest, runs on PHP). That alone gives me a certain amount of respect for it. Other than that, I have no opinion on it.