I’ve recently come across this awesome design sketching software. Its by Autodesk (the makers of AutoCad, but also a whole suite of awesome tools for DIYers, makers and small shops).
Sketchbook works on all your desktop and mobile devices, has a massive suite of tools and guides for creative and design application. I’m just skimming the surface at the moment, but for US$30/yr, its a steal. Yes, I did pay for the Pro version.
Check it out if you are looking for a drawing app.