Artrage: nice and painty.
Apart from the excellent paintiness of this app, you can export your small painting as a script, and if you buy the Windows or Mac version (for a modest fee) you can play it back at a big resolution. And save it big without pixellating. This one started out at 1024 x 768 pixels and ended up at 6667 x 5000. It just needed a little clean-up where the eraser hadn't quite done the job, then it was print ready.
Happy Squirrell. ArtRage rocks.