Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Feather Icons - Simply beautiful open source icons
IconStore - Free icon packs by first-class designers
Flat Icons - 20,000 unique, customizable icons with free lifetime updates
Noun Project for Mac - Search nearly a million icons. Now in color!
Streamline icons - The world’s largest icon pack library - 100k icons and illustrations.
IconBros - Free and continuously growing high quality icons