I'm actually the lead web guy over at SignNow and wrote our web app. I'm happy to try and answer any questions you have about the various methods we use.
Since you mentioned FlashCanvas specifically, FlashCanvas is a Flash-based HTML5 Canvas fallback solution that acts as a relatively transparent shim for browsers who do not support HTML5 Canvas natively (IE8, in our case). For all other browsers, we use the native canvas to for drawing signatures.
You can find more about FlashCanvas here: http://flashcanvas.net/
Also, I wanted to note that the licenses for any library we use are included in the comments in lib.js. Not all the code in lib.js is open source, including our in-house drawing library which is proprietary as is everything in app.js