Moqups is a nifty HTML5 App used to create wireframes, mockups or UI concepts, prototypes depending on how you like to call them. They’ve tried to make things simple and fairly intuitive so you can unleash your creativity without any obstacles. Moqups is built on open standards, striving to provide the best experience within the browser, without compromise.
There are 60+ handcrafted, crisp-looking SVG stencils (and more down the pipeline). You can export to PDF and PNG. It supports Auto-save. You can easily Undo, Redo, Cut, Copy and Paste. You can use Desktop-like keyboard shortcuts. It supports snap to objects/grid when manipulating objects as well. And it’s free.