I recently came across XWiki, a relatively new, Java-based Wiki. For anybody looking to deploy a new Wiki, this looks like a great option. It has an extensive feature list and seems to combine the best features of other Wiki engines as well as add a few unique Features of its own, for example scripting using Groovy or Velocity, PDF export, and others. It has a plugin API and supports RSS. The website layout is very clean. Definitely worth checking out.