Nucleo - Commercial vector icons and accompanying apps to manage and customize them.
The Noun Project - Creating, Sharing and Celebrating the World's Visual Language
Endless Icons - A collection of free flat icons and creative stuff
silhoutteAC - More than 47,000 vector silhouettes for free
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!
Zwicon - A simple, carefully designed collection of icons