DokuWiki - DokuWiki is a simple to use and highly versatile Open Source wiki software that doesn't require a database.
Bloomfire - Let Bloomfire help you get organized! Organize your content, build your company knowledge base and help your employees to be more successful.
Banana Dance Wiki/CMS - Free open source software combining the best of wiki and CMS with community building-features.
Confluence - Confluence is content collaboration software that changes how modern teams work
TiddlyWiki - a non-linear personal web notebook
Tettra - Tettra is a company wiki that helps teams manage and share organizational knowledge.