I've recently discovered the sketching software Al.chemy for myself. Sometimes I paint on the base of a collage of plenty of gradients, structures, and textures, then draw a sketch on top and develop a painting from it, but Al.chemy can do all that in a much more random way, plus nice recording functions. Here are some of the sketches I created with Al.chemy; the cityscape has had a little touch-up in Photoshop.
Al.chemy is an open source software and it's free to use. You can download it on http://al.chemy.org/