Signature Pad is a JavaScript library for drawing smooth signatures. It's HTML5 canvas based and uses variable width Bézier curve interpolation based on Smoother Signatures post by Square. It works in all modern desktop and mobile browsers and doesn't depend on any external libraries. Demo works in desktop and mobile browsers. You can check out its source code for some tips on how to handle window resize and high DPI screens. You can also find more about the latter in HTML5 Rocks tutorial...