Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Pili Pop - Great language learning method for kids
Violentmonkey - Violentmonkey is a userscript manager to support running userscripts in web pages.
Language Learning with Netflix - Use movies as study material for language learning
GreaseKit - GreaseKit is a SIMBL plugin, that adds user scripting to Safari, Mailplane, Diet Pibb.
Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.