Just about anything. Intranet wikis can replace a good deal of management overhead IMO.
Are busy Wikis a nightmare to keep up with? Since anyone can pretty much edit on site documents, it seems there would be a lot of comments and edits that would need to be reversed, research for validity, or corrected on a daily basis.
I'd require membership but again...all depends on your intended use.
What Wiki software offers the best ease of use, performance and largest active user base? (pay or free – does not matter – I would want the best)