Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Font Awesome - Font Awesome makes it easy to add vector icons and social logos to your website. And version 5 is redesigned and built from the ground up!
Feather Icons - Simply beautiful open source icons
The Noun Project - Creating, Sharing and Celebrating the World's Visual Language
IconStore - Free icon packs by first-class designers
IcoMoon - IcoMoon is an icon management tool that is build to provide iconography.
Flat Icons - 20,000 unique, customizable icons with free lifetime updates