Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Iconoir - A diverse and open-source library with 900+ icons
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1000 Detailed Creative Icons - Carefully designed icons with tons of fine details
IconStore - Free icon packs by first-class designers
LineIcons - 2000+ line icons for designers and developers
Flat Icons - 20,000 unique, customizable icons with free lifetime updates