Balsamiq Wireframes is a small graphical tool to sketch out user interfaces, for websites and web / desktop / mobile applications.
We focus on the ideation phase, providing you with a clean tool that lets you 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 your wireframes.
Wireframes is sold as a web application (Balsamiq Cloud), a Desktop application, and integrated with different wikis and bug trackers (compare all versions). You can try out Balsamiq Cloud for free, or download a free desktop trial. You can also easily access older versions.