MediaWiki and Wikipedia
The development of MediaWiki closely follows the development of Wikipedia, and vice versa.
Both MediaWiki and Wikipedia focus primarily on text. A number of text-based tools have been replicated from a standard version control system (where MediaWiki came from) to Wikipedia: user access controls (page protection, account creation restriction), revision control (page histories), diffs (diffs), pre-submit review (FlaggedRevs), etc. MediaWiki's code review tools still boast certain features such as git blame (attribution on a per-line level) that Wikipedia does not yet have.
Both are projects that have been hacked upon by volunteers (and paid editors!) for over a decade. These people have different motivations, different skill levels, and different commands of the English language. Consequently, parts of both MediaWiki and Wikipedia are impressive, while other parts are dysfunctional and amateurish.
Both projects also adhere strongly to a free content/open content model.