DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
Zim Wiki - Zim is a graphical text editor used to maintain a collection of wiki pages. Each page can contain links to other pages, simple formatting and images.
Banana Dance Wiki/CMS - Free open source software combining the best of wiki and CMS with community building-features.
Pepperminty Wiki - An entire wiki in a single PHP file. Intended for small - medium sized wikis.
TiddlyWiki - a non-linear personal web notebook
Teamemo - Modern wiki software as a service for intranets and knowledge management.