Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Space Icons - 77 hand-drawn space icons in multiple graphic versions
Feather Icons - Simply beautiful open source icons
IconStore - Free icon packs by first-class designers
Iconjar - Store all your icons in one place, just a click away
Flat Icons - 20,000 unique, customizable icons with free lifetime updates
Material Design Icons - 750 Free open-source glyphs by Google