ProductWiki.com runs on a proprietary wiki solution developed specifically for the task by the ProductWiki team. We've been evolving the software continuously over the last 2 years since our launch. It is built on the .NET framework and runs on a MySQL DB. We use AJAX.NET Professional for the AJAX stuff, and the text editor is created by FCKEditor.
Currently, there is only one other site that uses the platform: MadWhips.