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!
Streamline icons - The world’s largest icon pack library - 100k icons and illustrations.
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.
Flat Icons - 20,000 unique, customizable icons with free lifetime updates