The MediaWiki software that runs Wikipedia is under constant development. Do you want to see the latest, greatest features? Then head over to test.wikipedia.org, where the development branch is tested. But be careful. That wiki runs in debugging mode, so even the smallest problem in the code might cause it to spew out error messages. Take a look at the list of features under development in the MediaWiki roadmap. MediaWiki already is one of the most feature-rich wiki engines; see the MediaWiki feature list. MediaWiki and its dependencies are open source, so if you are the inquisitive type, please take a look at the code and help improve it.