Balsamiq offers a tool called "Balsamiq Wireframes", which they support passionately and has a big community around it. Balsamiq Wireframes is a small graphical tool to sketch out user interfaces for websites and web, desktop and mobile applications. It focuses on the ideation phase, providing the user with a clean tool that lets the user get in the flow and stay focused on structure rather than colors and icons. Wireframes provides enough interactivity to replace prototypes most of the time, and makes it easy to collaborate and get feedback on one's eireframes. It have extensive docs, and improve it's software with frequent new releases and provide support via multiple channels.