If you've ever wanted to roll your own wiki and are not intimidated by having to deal with some of the technical underpinnings that come with an industrial-strength wiki solution, you might want to take a look at MoinMoin. This open source, Python-based wiki features revision tracking, wiki formatting, data import and export, and more.
Installing MoinMoin is not difficult: I already happened to have the Python scripting language installed, so I just unpacked the MoinMoin archive and ran wikiserver.py. That was it: MoinMoin started, and I was able to access it on http://localhost:8080 less than a minute after downloading it. (That said, anyone who thinks Python is just a snake or a bunch of funny Englishmen might have a harder time.) The console window remains open as long as MoinMoin is running, displaying a Matrix-esque stream of logging information showing exactly what's going on in your wiki. If it gets in the way, you can always minimize it to your system tray using Minime. See all software downloads.
Moin's layout is complex due to its sheer amount of features. While MoinMoin is powerful and extensively documented, there's not a lot of hand-holding: The system assumes you already know what you're doing. For starters, MoinMoin is available only as a tar.gz archive, so you'll have to use an application like 7-zip or Total Commander to extract it. After I got it up and running, I promptly locked myself out of the main page: I tested the page import/export functionality by exporting the Features page from the official MoinMoin wiki and importing it into my own local instance. Sadly, I didn't notice the original page had an ACL (access control list) directive at the top, which made it editable only by the MoinCoreTeamGroup user group (that didn't even exist on my local wiki). MoinMoin gave me no warnings, imported the page smoothly, and that was that. I was stuck with a completely static home page that I couldn't edit or revert in any way.
Fortunately, pages are stored in plain text: I ended up having to dig into the directory tree manually (using trusty Total Commander) and remove the offending page. I then restarted MoinMoin and reimported the page, this time sans the offending ACL line. Problem solved: I was now able to edit the page. See also: TiddlyWiki review
Like most serious wikis, MoinMoin automatically saves page revisions. You can easily compare revisions, even if you don't have permission to edit the page or revert it to a previous revision. MoinMoin also has a vibrant market of extensions and themes, although if you're looking for a wiki with dozens of themes, you might be better off with DokuWiki. As it stands, I counted ten themes on the official MoinMoin Theme Market. Not a staggering variety, but you can always make your own.