I was looking at wikis recently and thought I'd list out the ASP.NET wiki's I've noticed. Obviously there could be more, but here are the one's I've seen:

• FlexWiki -- been around for a while, easy to install & use.
• ScrewTurn -- has gotten some good buzz recently, seems the most active recently w/ a number of interesting features.
• Perspective -- I recently noticed this. Has a WYSIWYG editor (which I think is good for corporate adoption). You can also attach & embed files (e.g. Microsoft Office Docs) & images, & search across them. Seems like it might be complicated to set up & install.

I believe they all can do authentication/authorization through NTFS and/or LDAP, so you can administer the permissions like you would any other IIS web site.

Here is a WikiMatrix comparison page of the above three wikis, although I don't believe their information is completely current.