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!
Brands of the World - Open source database of brands vector logos
Nucleo - Commercial vector icons and accompanying apps to manage and customize them.
LogoSear.ch - Instant search through 200K+ SVG logos from 100+ sources
IcoMoon - IcoMoon is an icon management tool that is build to provide iconography.
Flaticon - A database of free vector icons.