http://wagn.org/wagn/Introduction.
A WikiEngine. A reasonably clean implementation of transclusion, combined with the concept of taking any two existing “cards” and creating a card for their intersection. The topics of cards are like tags; for instance, if you have a card “Dell” about Dell Computer, and a card “Labor” about “labor issues”, then to talk about labor issues related to Dell Computer, you make a card Dell+labor.
Written in Ruby (on Rails).
See also FacetWiki.