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!
The Noun Project - Creating, Sharing and Celebrating the World's Visual Language
Flaticon - A database of free vector icons.
Nucleo - Commercial vector icons and accompanying apps to manage and customize them.
Fontello - Tool to build custom fonts with icons.
IcoMoon - IcoMoon is an icon management tool that is build to provide iconography.