Wikia is a little bit like Wikipedia, but it's actually a white-paper service that lets site owners add wiki elements to their sites and brand them to match. The site has enabled many to create niche community microsites inside their own services covering all sorts of topics. It also lets users dig in a little deeper than is allowed in Wikipedia's governed environment while still using the same tools.
Since Wikia does all the hosting, users who want to create their own wikis don't have to install anything or pay for servers, hosting, and so forth.
Winner: Wikia (Wikia.com)