Wireframe.cc A simple wireframing tool that doesn't get in your way. Create and share simple wireframes of websites and mobile apps
Balsamiq Cloud Effortless UI sketching. Capture your ideas, collaborate on your designs, get everyone on board
Whimsical Wireframes Instant wireframing at your fingertips. Rich library of configurable elements like buttons, inputs, checkboxes, and more
Sketchize Just choose any of the sketch sheets that fits your project, print it out and start to sketch your dream