Universal Edit Button от travis_wikihow, Andreas Gohr

Using this extension, an icon will appear in the address bar (similar to the RSS auto-discovery button) when you visit a wiki website that allows you to edit the page.

This works on popular sites like Wikipedia, AboutUs.com, wikiHow and Wikia and also on most of the major wikis including MediaWiki, DokuWiki, TWiki, PHPWiki, MoinMoin and many more.