JSPWiki is an open source Java-based WikiEngine. It has a very modular architecture and thus has been used in many projects to provide a wiki component. It is also a development platform, thanks to its simple-to-use plugin architecture.
JSPWiki is quite a powerful wiki on its own as well, functions well with existing infrastructure, has spam filtering built-in and does not require a database!
As of version 2.8, JSPWiki is licensed under the Apache License.