Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Anima App - Design, get feedback, convert to code, publish, iterate.
Feather Icons - Simply beautiful open source icons
Sketch Repo - Collection of resources for anyone who uses Sketch
IconStore - Free icon packs by first-class designers
Auto-Layout for Sketch - Responsive design for Sketch
Flat Icons - 20,000 unique, customizable icons with free lifetime updates