Really depends a lot on your needs. A simple wiki like MoinMoin doesn't offer a lot of features, but is correspondingly care-free from a maintenance perspective. A more complex wiki such as MediaWiki, twiki, or Confluence adds a lot of extensibility and (through plug-ins, or directly integrated) usability niceties, but also some management overhead. If you're a development shop, I'd also suggest that you also look at Trac, which combines integrated bug tracking, blogging, time tracking (with a plug-in), and wiki in a single package.