Open Peeps - A hand-drawn illustration library.
Thunkable - Powerful but easy to use, drag-and-drop mobile app builder.
Control Illustrations - 108 free flat illustrations with customizable characters
BuildFire - BuildFire is the easiest way for small businesses to build a mobile app in a matter of minutes for iOS and Android.
Craftwork - A collection of User Interface resources made by Craftwork
MIT App Inventor - App Inventor is a cloud-based tool, which means you can create apps for phones or tablets right in your web browser.