Balsamiq a cool GUI mockup tool

Balsamiq Mockups for Google Drive allows you to create, edit and share sketch-quality wireframes, all within Drive.

Balsamiq Mockups is the easiest, fastest, most fun, best supported wireframing tool on the Internet. The integration with Google Drive is pretty slick: you can create or edit mockup projects directly from Google Drive, all within the browser. You can even collaborate on your wireframes with others, in real time!

At the end of your fully-functional 30-day trial period, you will be able to view existing mockups, but you will not be able to edit or create new mockups unless you sign up for a paid plan. To do so, you have to sign up to either a no-risk $5/month or a $50/year subscription plan. You can stop and restart your subscription at any time.

http://www.balsamiq.com/