Recently I've been playing a lot with two free pieces of software. Both function largely as one-trick ponies, but their methodology makes them extremely fun to dive into.
First up is Alchemy. It's almost like drawing in real-time with a lasso when you use it right, and the mirror function is incredibly fun for quick bilaterally symmetrical drawings. Check it out at al.chemy.org
Second up is Harmony, an awesome little ditty of HTML5. It's got an inky, bleedy feel to it, far better than anything I've managed to get in Photoshop! It's great for relaxation, since you can draw lazy and it'll still look appealing. In my own humble opinion, the horse I made with it is the first good drawing I've ever freehanded with a mouse. If you've never tried drawing with a mouse, it's like drawing with the wrong hand with the wrong fingers. But Harmony still rocks! http://mrdoob.com/projects/harmony/
Six pages left on my current sketchbook. It's always so triumphant to finish one, but it's also so tedious to scan all my pics in at the end...soooooo tediouuuuuus....I think I subconsciously forgot to bring a pencil today to delay the inevitable :P