XWiki is both an open source enterprise wiki software and a platform for quickly writing simple collaborative web applications. It's written in Java with a design emphasis on extensibility.
XWiki is a free software platform written in Java based on the wiki principles, under the LGPL license. In addition to being a full-featured wiki, it is also a second generation wiki allowing effortless development of collaborative web applications. On top of this platform a plethora of applications are developed, targeted mainly on aiding enterprise-level needs.
XWiki Enterprise, the enterprise wiki edition, includes WYSIWYG editing, OpenOffice based document import/export, semantic annotations and tagging, and advanced permissions management.