Harnessing Mixins in SCSS. Let's Start With What Is @mixin. | by Nathasha | UX Planet
SASS Mixins - Codepad
CSS Shadow Effects That Make a Statement
Layered box-shadows via Sass mixin
GitHub - at-import/sassytextshadow: Sassy Text Shadow is a Sass mixin that calculates convoluted curvy shadows for css3 text-shadow. It can also be used for box-shadow if you're into that kind of thing.
Another SCSS mixins library - Codepad
What is wrong with my box-shadow SASS @Mixin? - Stack Overflow