Creature sketching using a fantastic open source and free drawing application called MyPaint.
Running this application on my laptop (running Linux Mint), very impressed with it so far. Its very stable, has pressure sensitivity to my Wacom drawing pen and has tons of plugins and brushes. If youre running windows, they have a windows installer also. Really cool stuff. I now have 2 open-source and free applications that I always use for graphic design, InkScape (vector drawing) and MyPaint (painting and sketching).