ProjectForum

ProjectForum is a commercially supported, high-quality wiki collaboration server for small and medium workgroups.

It runs securely on an organization's own network, keeping important information safely in-house and protected. Supporting multiple workgroups from a single server, it features full version control, secure authentication options, document management, full branding support, and much more.
Unlike other wiki software, ProjectForum is affordable, easy to set up, easy to learn, and easy to use--for any workgroup. ProjectForum can be downloaded and set up on an internal network in minutes by anyone, without the dependencies, complex installation, and maintenance headaches inherent in most wiki software.
ProjectForum is free to use with unlimited users working on one wiki. The licensing schedule for multiple wikis can be found here.