First there was Everything2, a Perl-MySQL Web Content-Management System to create a flexible system of entering, linking, and retrieving information. I remember first discovering this system when I still read Slashdot sooooooo long ago. Cool idea, less concerned with being an authoritative reference and more about being a sounding board for anyone who is interested in writing about a given topic. Everything2 was developed by the same guys who developed Slashdot which is why the Perl/MySQL solution was used. It offers a simple search mechanism for discovering nodes.
Next came Wikipedia, a multilingual, Web-based, free content encyclopedia project whose articles provide links to guide the user to related pages with additional information. Wikipedia was born out of Nupedia which was an effort to create a new encyclopedia via an elaborate system of peer review that required highly qualified contributors. Wikipedia threw out most of the formalities of qualifications and peer review became a staple due to the high amount of collaboration. Still, Wikipedia is most interested in information that is worthy of notice. Wikipedia also offers a primitive search mechanism for locating information. Wikipedia also prides itself on the anonymity of its content creators.
About.com provides it's own information that is managed by its guides, people who have the credentials and experience to back up knowledge. Guides must also have professional writing experience in your area of expertise. It's a unique set of qualifications, but About.com actually pays its guides to author the content. About.com offers a lot of information but it is owned by the New York Times Company which gives it a really commercialized feel. Where Wikipedia definitely has an encyclopedic approach to its information, About.com has a very consumeristic bend to its content.
[Interestingly, a company named Powerset developed its own search technology focused on aggregating, summarizing and navigating information and it's first focus to showcase its technology was a combination of content from Wikipedia and Freebase. (Earlier this month, it was announced that Microsoft acquired Powerset.)]
Another player in this space is Squidoo offering its own set of information authored by anyone. Squidoo has mulitple goals listed on its website including it's goal as a platform is to bring the power of recommendation to search; it's goal as a co-op is to pay as much money as we can to our lensmasters and to charity, and it's goal as a community is to have fun along the way, and meet new ideas and the people behind them. So it pays its authors and charities and promotes having fun authoring content and recommending it.
The first offering from Google was GoogleBase as a way to describe your information to make it as easy as possible for people to find when they search. In other words, enter your information and make it part of the Google-verse. Oddly, this seems to overlap with Google Knol.
Now comes Google Knol, a system for creating authoritative articles about a specific topic. Knol seems to be somewhere in between Everything2 and Wikipedia but it removes anonymity from the picture by requiring information creators to have a Google account. Knol is yet another way for Google to exploit its business of content-targeted ads.
So how do these information systems differ? IMO they really don't differ much in what they seek to provide, they only differ in their implementation. Each one seems to store and make it's information available in it's own, unique way. So to compare and contrast each one, I searched for the string 'Boulder, Colorado'. Below are links to the results.
Boulder, Colorado on Everything2 returned a spartan amount of information.
Boulder, Colorado on Wikipedia produced the most information out of any of these systems.
Boulder, Colorado on About.com.
Boulder, Colorado on Freebase.
Boulder, Colorado on Squidoo produced no content.
Boulder, Colorado on GoogleBase produced the results in the same format used by Google Shopping and other Google properties.
Boulder, Colorado on Google Knol rendered nothing.
The bottom line is that all of the sites I mention here are focused on organizing and providing information. The quality of the information must be good, but that's only a contributing factor, IMO. If the quality of the information is good, the real differentiating factor is the value-add around the edges. And right now the biggest value-add seems to be how the information is offered to be discovered by users. They all have their own way of attacking the problem.
Wikipedia offers the best content quality hands down. This is surely due to the high amount of collaboration at Wikipedia and the Wikipedia community's ability to police it's content so incredibly well. Each entry is typically fairly well-rounded and has been contributed to by multiple people - the wisdom of the crowd at its finest.
Freebase is more interesting to me because it offers an API for accessing the data (and because I'm a software engineer, I'm biased on that front) but Freebase can't shake a stick at the content offered by Wikipedia. Freebase is also not the best at presentation.
Even though Powerset is not a content creator, it's ability to aggregate data and present it to users in a more meaningful manner is probably the most compelling just because it's the most usable by far. But not that many people have even heard of Powerset and how that it's been swallowed by Microsoft, it may never be heard from again (unless Microsoft leaves it untouched to continue to do what it does).
Google certainly has the most marketing power and its dominance in so many other web properties gives it a leg up. But it's content breadth and depth is sorely lacking currently. Maybe it will catch up over time.
Which one will prevail? Your guess is as good as mine. Competition is a good thing ;-).