What software does Wikia use and can I have that for my own hosted Wiki?
Solution
You can find Wikia's customizations and extensions to MediaWiki at their dev wiki.
They keep most of their tweaked code in a publicly-accessible SVN server, linked there.
OTHER TIPS
AFAIK, Wikia uses a modified version of the standard Mediawiki distribution that they've been augmenting over the years.
Their business model is to host Wikis (and generate ad revenue) but also to attract users with features not available in standard MediaWiki. I don't think they're offering the software for free, and I doubt that they're selling it at the moment.
Wikia uses the latest version of the MediaWiki software (MediaWiki 1.13.4), but with many of the extensions. The advanced features you see are their hard work at adding a "skin" (called Monaco) of css and javascript over the top. If you ask politely, they will give you a copy.
Their source code is in a public GitHub repository.
If you want to download it and use it on your own go to the latest "release-*" branch.