Wikis are great ways to structure knowledge together with other people. Wiki means fast in Hawaiian, which is for a reason. I've written a little overview of free wiki software, now it's time for free wiki hosting. Unfortunately spammers have also found ways to use wikis, mainly for black hat SEO linkbuilding purposes. This makes it pretty hard to keep a community wiki running.
Wikia is by far the biggest, it was founded by the founder of Wikipedia. Naturally it's running MediaWiki. It's also quite full of advertising. The plus side is that you can download database dumps which would make it easy to set up shop elsewhere, which is exactly what happened with Hitchwiki (although Wikia insisted on keeping the old content and trying to retain editors).
Wikidot is an interesting option. They let you earn some of the advertising dollars.
Localwiki is a great project, a non profit 501(c)(3), started by the folks who started the very successful DavisWiki. The idea is to help local communities through wikis. So if you want to run a local community wiki this could be a great place for you. Unfortunately the platform offered is not based on MediaWiki.
Cheap MediaWiki hosting
It's also possible to keep full control over your wiki by setting up an installation with hosting providers. The following offer convenient ways of installing MediaWiki:
- Dreamhost offers one-click MediaWiki installation through the admin panel, starting from 8.95 US$ per month.
- Bluehost is another good shared hosting company, also with convenient MediaWiki installation and slightly cheaper, especially if you pay ahead for 3 years.
- Wikipedia has an extended overview of wiki hosting services.