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!
Vivid JS - A highly customizable, open source SVG icons library
Flaticon - A database of free vector icons.
Line Awesome - Same icons as Font Awesome, but in a modern outline style.
Fontello - Tool to build custom fonts with icons.