Don't forget Confluence
Don't forget Confluence as well: it's a really nice wiki system (we use it at work). It's now all nicely menu/toolbar driven, but if you are feeling l33t you can still use the keyboard shortcuts and markup while writing your documents, and *impressively quickly*, too. If you like Markdown-like syntax, you'll like it.
(Admittedly, I'm not the one who is paying for it, so whether it's really worth the money as opposed to, say, MediaWiki, is perhaps another question, but it is well used in the techie parts of our organisation and acts as a genuinely useful shared knowledge base for us.)