Robin Good: With the free Keynotopia Mockup Bundle, you can draft, edit and sketch highly-detailed elements of a user interface while using only Apple Keynote (but don't fret. There are libraries and themes also for PowerPoint and other presentation tools).

The Keynotopia Mockup Bundle includes:
-> Over 1,000 vector user interface components designed entirely in Apple Keynote. The following templates are included in the bundle:
->> iPhone Mockup Templates
->> iPad Mockup Templates
->> Android Mockup Templates
->> BlackBerry Mockup Templates
->> Web Application Mockup Templates
->> Facebook Mockup Templates
->> OS X Mockup templates
->> Windows 7 Mockup Templates
->> Windows Phone Mockup Templates

"Keynotopia is the largest collection of user interface design templates that enable you to prototype and test your app ideas in 30 minutes or less using Apple Keynote, Microsoft PowerPoint, or OpenOffice Impress.

The templates include thousands of wireframe and high fidelity vector UI components, meticulously designed from scratch in Keynote, Powerpoint and OpenOffice, and fully editable and customizable without needing additional design tools."

The Keynote Mockup Templates Bundle is available in exchange for a simple tweet (normally they are $49).

Find out more: 

Templates and Themese Gallery: