MediaWiki is the half-alright Wiki CMS system used by this Wiki, and by the Gentoo Wiki. It's pretty buggy, and poorly coded, but it's alright to use some versions.
Verison v1.9.1 is totally and utteryly broken. It's just completely destroyed if your site is, or plans to be, available to IPv6 users.
Verison v1.8.x is the last stable version of MediaWiki. This version is not utterly broken in so many ways like v1.9.x is, and it also works with IPv6.