Alchemy is a very nice piece of software that I’ve found in the past months that is basically a sketch environment with several tools oriented to shape, lines exploration. I find it particularly useful when the brain is tired of thinking, there are tools that just throw chaotic lines and shapes around that can be worked on another software such as Photoshop, Painter, etc. Its released under GNU license. Alchemy also has a pretty decent documentation.
A couple doodles I did in Alchemy below: