FYI I started a GoDaddy VDS (Virtual Dedicated Server) wiki at

http://godaddyvds.wikispaces.com/

My $0.02 on wikispaces: you can create a free, basic wiki there. It's not bad, although slow. I'd like to be able to set my preferences so that when I edit a page it starts with the text mode editor instead of the WYSIWYG editor, b/c it takes considerably longer to edit a page using the WYSIWYG editor than the text editor. Also, the lack of any AJAX on the site is noticeable...everything you do involves a page load or page refresh (this is why the WYSIWYG editor is so slow, since it uses a number of popup windows).

So, if your audience is non-technical and/or you want a hosted solution, Wikispaces is fine. Or try StikiPad which is pretty similar (thx Scott).

Otherwise, if you have your own Linux host and/or you're more of a techie? Pick a different wiki. :) Moin Moin is easy to install, Giki is really easy to install (no search feature though), TWiki is easy enough...there are a lot of wikis you can compare. A number of hosting providers (e.g. DreamHost) offer various wikis as one-click installs, which is even easier. :)  And of course there are a few ASP.NET Wikis you can install, too.