Balsamiq Wireframes

Balsamiq Wireframes is a rapid low-fidelity UI wireframing tool that reproduces the experience of sketching on a notepad or whiteboard, but using a computer.

It really forces you to focus on structure and content, avoiding lengthy discussions about colors and details that should come later in the process.

Wireframes is FAST: you will generate more ideas, so you can throw out the bad ones and discover the best solutions.