Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Hue - Unlimited design for startups to enterprises
Feather Icons - Simply beautiful open source icons
IconStore - Free icon packs by first-class designers
Artboard Studio - Online graphic design app mainly focused on product mockups.
Flat Icons - 20,000 unique, customizable icons with free lifetime updates
IconBros - Free and continuously growing high quality icons