Not only are there other projects based on the Wikimedia software out there, there are even companies, such as Wikia, offering ready to use Wiki-Systems for theme based content-generation.
Alternate History: A wiki collecting storylines of fictional alternative histories of Earth, a literature genre sparked in the 20th century assuming that if certain events went only a little bit different from the way, they happened in our world, Earth might look totally different. There are numerous examples around, some as fictional as SciFi, but some with interesting thought plays, e.g. if Stalin would have discovered the nuclear bomb before the Americans and how that would have influenced WWII.
If people have already enough free time and energy to create wikis with more than 60000 articles on a fantasy world known from books and movies, I wonder where the border, the limit, the next frontier lies for the internet and the way we, people, can use it to make not only our own lives better, but smarter.
Because, whatever the reason is, that people invest time, most people (following the homo economicus principle) must find it reasonable and worthwile to invest time and energy. Whether it is esteem (Maslow’s hierarchy of needs) or self-actualization, people do it. I wonder where the scientific wikis are, the physics, the cross-university-wikis, the research ones. Maybe I haven’t looked close enough…
Myself, together with several good friends, have launched the RiC Wiki (not at Wikia) several years ago, featuring information about the growing music scene in China, both from a listeners approach, and from a scientific approach (see history, articles).