Fontawesome: easy to use and install (use CDN or import files) icon library
Font Awesome, the iconic font and CSS toolkit
Budicons: beautiful icon set; comes in outine, shaded, and filled.
The Noun Project: if you need a wide variety of icons to select from or get some inspiration, come here.
Free Flat Icons: what the name says
56,400 Free Icons - The Largest Icon Pack Ever
Flat Graphic Design Icons (Download): a gift from InvisionApp
Download 36 flat graphic design icons-free! - InVision Blog
Flat Icons Pack (Download): has popular app icons
Custom Adobe CC Icons: just some cute Adobe icons