Possibly far more features than any other Wiki available, including embedded scripting within Wiki documents (a huge plus for me) and embedded Hibernation SQL/fulltext engine. Unfortunately, using most of these features in XWiki are undocumented for now. Definitely worth a look if you want something with a feature set on par with (or vastly surpasses) TWiki, but faster (Java-based, runs under Tomcat engine) and more dynamic like Zope.
An open, cross-platform journaling program.
A scientific plotting package.