I highly recommend Keynotopia for designing interface mockups for web and mobile apps

Keynotopia is a user interface design toolkit that enables you to use Apple Keynote or Microsoft PowerPoint to prototype, test and demo your application ideas quickly and cheaply, without doing any design work or writing a single line of code.

It includes thousands of wireframe and high fidelity vector user interface components and royalty free icons for mobile, web and desktop apps, all designed from scratch in Keynote and PowerPoint, and can be edited and customized without needing any design tools.

Keynotopia is used by 40,000 entrepreneurs, designers, and developers in more than 80 countries.