IconStore - Free icon packs by first-class designers
The Noun Project - Creating, Sharing and Celebrating the World's Visual Language
omg-img - Insert icons directly into your HTML just by coding
Freepik - More than a million free vectors, PSD, photos and free icons. Exclusive freebies and all graphic resources that you need for your projects
Mosaic - Mosaic provides brands with solutions to store and categorize their digital graphic and photography files for quick and easy retrieval.
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!