XWiki looks like a very interesting project. The main idea seems to be to provide a “platform” on which a Wiki is built as an example of the kinds of applications it suits. Extension and development is by in-page scripting (e.g. Velocity) and Java plugins which interact with a fundamental API and data model.
I’m really tempted to move the next version of Friki in this direction. Friki already has arguably better text markup flexibility, but the ability to build “applications” on top of it would be an extra level of power.