Hand drawing/sketching

Cardboard Modeling – Instructional site for foam core and cardboard modeling by Joep Frens

Drawing Ideas – Book and resources for sketching/drawing for interaction designers.  By Mark Baskinger and William Bardell

Sketchnote Army – sketchnote examples and inspiration

2D Prototyping Tools

PimpMyDrawing – Free vector graphics of people

Flintio – App prototyping tools for designers

Marvel – Simple design, prototyping and collaboration tool for designers

InVision – prototyping, collaboration & workflow platform

Principle – prototyping with animated and interactive user interfaces

Wytepad – reusable physical acrylic boards for sketching interactive prototypes (use together with Marvel for more mojo)

MockuPhone – screen mockups of mobile devices

Keynotopia – tool for using Keynote or Powerpoint as prototyping tools for creating mobile, web and desktop UI mockups

Drawing Software

Cacoo – cloud-based diagramming tool with social editing features

InkScape – free vector drawing software (similar to Adobe Illustrator (AI), however AI is more widely used)