sketching on the go
Krita is a free sketching and painting program that's great for things like concept art, texture or matte painting, illustrations and comics, and more.
Krita Portable works best with the PortableApps.com Platform
Krita is a free and open source painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry. Krita has been in development for over 10 years and has had an explosion in growth recently. It offers many common and innovative features to help the amateur and professional alike including an intuitive UI, brush stabilizers, a pop-up palette, customizable brush engines, seamless textures and patters, layers, assistance tools, PSD support, and more. Learn more about Krita...
Note that Krita may temporarily disable Aero when running on Windows Vista/7, switching the OS into the Windows Basic theme.