Just wanted to remind everyone that we distribute an open source version of Wikidot, at Wikidot.org. Piotr Gabryjeluk (Gabrys) is the maintainer of this great package of software.
In the last months, Piotr has made Wikidot.org much easier to install, and you can download it to use for single-wiki cases, just like Mediawiki (but it's much easier to install and configure). And then you can flip a setting and get a wiki farm, just like Wikidot.com.
Wikidot.org (which is really just Wikidot.com with open source packaging) is written in PHP. If you're a developer and you ever felt like hacking Wikidot, fixing some of those bugs, or extending any aspect, take a trip over to www.wikidot.org now.