Iconmonstr
Iconmonstr is free, monstrously big and continuously growing source of simple icons created by Alexander Kahlkopf, a senior designer from Germany with over 15 years of professional experience in icon design.
Icon8
Icon8 is a flat icon,They have team of 4 designers, 5 developers, and a content strategist. No two people live in the same city and we’re spread from New York to Melbourne.
Font Awesome
Font Awesome is a font and icon toolkit based on CSS and LESS. It was made by Dave Gandy for use with Twitter Bootstrap, and later was incorporated into the BootstrapCDN. Font Awesome has a 20% market share among those websites which use third-party Font Scripts on their platform, ranking it second place after Google Fonts.
Font Awesome 5 was released on December 7, 2017 with 1,278 icons[3]. Version 5 comes in two packages: Font Awesome Free and the proprietary Font Awesome Pro (available for a fee). The free versions (all releases up to 4 and the free version for 5) are available under SIL Open Font License 1.1, Creative Commons Attribution 4.0, and MIT License.
http://fortawesome.github.io/Font-Awesome/icons/
Flat Icon
Flaticon is the largest search engine of free icons in the world. Flaticon offers users, high quality graphic designs: totally editable vectors carefully selected by our design team in order to provide our users with great content that can be used in both personal and commercial projects.
Google Icon
Google Material icon is an adaptable system of guidelines, components, and tools that support the best practices of user interface design. Backed by open-source code, Material streamlines collaboration between designers and developers, and helps teams quickly build beautiful products.
https://www.google.com/design/icons/
Endlessicons
Endless Studio Endless Icon is a project bulid by Min Kim who runs Email-Gallery, App Monstr, and Endless Photos. I created it to share some of the icons and creative stuff with the design community. Cheers!
Iconfinder
Iconfinder provides high-quality icons to millions of creative professionals. We are a small international team based in the lovely city of Copenhagen, with some of us working remotely. Together with a passionate community of icon designers, we are building the world’s most popular icon site.
Retro Vectors
Retro Vectors is a source of premium quality vintage vector stock files that are free for personal and commercial use. Victorian, 40’s, 50’s 60’s, 70’s & 80’s.
Freepik
Freepik offers users, high quality graphic designs: exclusive illustrations and graphic resources carefully selected by our design team in order to provide our users with great content that can be used in both personal and commercial projects.