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!
1000 Detailed Creative Icons - Carefully designed icons with tons of fine details
The Noun Project - Creating, Sharing and Celebrating the World's Visual Language
Icons Mind - The Biggest iOS 8, iOS 7 & Android Icons Pack
IcoMoon - IcoMoon is an icon management tool that is build to provide iconography.
IconStore - Free icon packs by first-class designers