Open Peeps - A hand-drawn illustration library.
Invision - Prototyping and collaboration for design teams
Control Illustrations - 108 free flat illustrations with customizable characters
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
Craftwork - A collection of User Interface resources made by Craftwork
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.