. The plants.
In the early days of the Internet, there was a similar phenomenon, the "everything has its home page" rule. Anyone with enough HTML savvy and a powerful enough interest in one particular subject could and often would create a site dedicated to it. This was largely how the early Web was forged. All the stuff we know and love today — e-commerce, social networking — that came later. Except pornography. Porn was there from the beginning.
With the advent of Wikipedia, however, the playing field changed. Rather than rely on a collection of sites each written by one person with questionable expertise, users could now find most of the information they needed in an article written, edited, and fact-checked by an entire user base of people... with questionable expertise! But all on one easy-to-remember site.
However, several factors kept Wikipedia from being a perfect replacement for the old system. A desire for greater detail on the topic than Wikipedia is willing to allow, for one. Schisms or differing schools of thought on the topic were another. Fan-made wikis sprang up to bring back the world of homemade sites with the added benefits of the wiki model. Wikis are also popular with open source software projects for documentation as wikis can be updated much more quickly than static pages. The "wiki-sphere" is becoming a vast depository of information at all levels of detail. The encyclopedic wikis are collecting a great breadth of topics at an increasingly shallow level of detail, and the topic-intensive wikis are gathering all the details.
Not all Wikis are on the major search engines, though. Corporations use wikis behind firewalls, the American CIA uses one to collate data among agents and analysts, and even publishers of dead-tree books use them to coordinate edits among authors, editors, and copy editors.
Fan-made wikis are usually made on wiki farms, such as Fandom (formerly known as Wikicities and Wikia).note Few are hosted on preexisting fansites, like Homestar Runner Wiki and TFWiki.net. Organizations such as NIWA (Nintendo Independent Wiki Alliance) have spoken out against the commercialization that takes place on wiki farms, and have encouraged fans to set up their websites, while The Anti-Wikia Alliance is against Wikia specifically.
Wikis can reach truly huge sizes. To put them up as candidates for a Doorstopper is an understatement. See the list of largest wikis. Also, due to the ever-increasing importance of wikis in modern internet culture as well as the ever-increasing amount of information stored in said wikis, several archival initiatives such as the Wiki Team by Archive Team (which in and on itself is a wiki) have sprung up to protect these valuable codexes of information from maladies such as expired domains and wiki farm shutdowns. Show them some love.
The "dead tree" equivalent of this trivia is The Magazine Rule.
