Open Peeps - A hand-drawn illustration library.
Spine - Spine is 2D animation software designed specifically for software and game development.
Control Illustrations - 108 free flat illustrations with customizable characters
Synfig Studio - Just another WordPress site
Handsome Illustrations - Create unique illustrations for websites & applications.
Unity 2D - Unity 2D is a powerful and feature-rich game design solution built from the ground up to fulfill every basic and advanced need of the developer.