Clip Art Panda strives to bring you the most comprehensive collection of clipart on the web.
Openclipart.org is a huge collection of clipart that is free even for unlimited commercial use. Giving away images is a great way to share their talent with the world that will come back around in a better form.
Freepik has modern, trendy, high-quality and exclusive illustrations and graphic resources.
Freepik graphics can be used for free if you give credit and a link back to the website.
They can be used without a credit link if you have a paid account.
Become a member of Creative Market, and get 6 free graphics-related downloads every week: graphics, fonts, WordPress themes, presentations, and more!