Open Source: Trac Wiki
Trac Wiki is a popular open source option. There's even a thread on another Stack Exchange site about converting Word documents to the Wiki Wiki format . Some major or famous Trac users include Red Hat, Django, Handbrake, and SourceForge.
Commercial: Confluence
Confluence is another popular option that Alex Korban talked about. A good example of its widespread use is the Application Server 7 JBoss software community documentation.
2015 Update - PressGang and Corilla
I will just add a quick update that I so strongly agree with this issue of detectable content and collaboration, which I worked on to solve it in several different projects.
PressGang (prototype)
Check out the PressGang CCMS project to understand what we did at Red Hat to create tools to solve this problem. The lead engineer made an end-to-end video that you can see on Vimeo, and I created Amazon's public AMI if you want to try It. It is not supported, but it is all open source. Check out the repository or turn on the community plug .
Corilla (product)
It was an amazing experience that was part of and inspired me to break out of Corilla , open source exactly the problems you are talking about. This will have both a commercial / corporate version and an open source community version. There is a beta version in which I would encourage you to participate in - the problems that you encounter will be good to form the solutions that are built,
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