Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Nucleo - Commercial vector icons and accompanying apps to manage and customize them.
Feather Icons - Simply beautiful open source icons
Icon Shelf - Icon Shelf is a free and open-source SVG icon manager for web developers.
IconStore - Free icon packs by first-class designers
Glacier Icons - A free icon set for your Mac
Flat Icons - 20,000 unique, customizable icons with free lifetime updates