The Noun Project - Creating, Sharing and Celebrating the World's Visual Language
Streamline icons - The world’s largest icon pack library - 100k icons and illustrations.
Nucleo - Commercial vector icons and accompanying apps to manage and customize them.
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!
Flat Icons - 20,000 unique, customizable icons with free lifetime updates
Freepik - More than a million free vectors, PSD, photos and free icons. Exclusive freebies and all graphic resources that you need for your projects