Radeox is a WikiWiki API for exchangeable rendering engines (just like JDBC) for Java. Wikis adhering to this standard interface can use other wiki engines. Wikis that use this standard interface can make their wiki rendering engine available to others.
Radeox RE is a WikiWiki RenderEngine? implementation written in Java. Easy to extend using a filter and macro plugin mechanism. Used by SnipSnap and released under the LGPL.