Zwicon - A simple, carefully designed collection of icons
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!
Material Icons Library - Free collection of 1000+ icons for design
Flaticon - A database of free vector icons.
Endless Icons - A collection of free flat icons and creative stuff
Fontello - Tool to build custom fonts with icons.