Realizer App
Free tool that lets you create professional prototypes for iPhone and iPad apps
UXPin
A superb range of paper prototyping, for both web and mobile
Android Honeycomb Stencils & Sketchsheet
Supremely useful Omnigraffle stencil and sketchsheets for the new Andriod OS
Mobile Wireframes
A Collection of Mobile Wireframes scanned from around the web
Sketching: Visual Thinking Power Tool
The talented Mike Rohde on how sketching can act as a powerful problem-solving and communication tool
iPhone Stencils
Free Omnigraffle iPhone (and iPad) stencils to mock up your prototypes
UX Pin
Paper prototyping - a complete system to document your website ideas on paper.
Connectasketch
Neat tool for turning sketches and wireframes into functional prototypes
