JugiPaint

JugiPaint is fantastic. It has a great default set of brushes that you can customize in quite a few ways, as well as the ability to create new brush tips straight from the canvas via selection (much like in Photoshop). Layers, layer masks and groups, all work wonderfully - you have the ability to set up clipping masks, lock opacity, change blending modes, etc. Vector layers with a pressure-sensitive brush add a nice scalable method of drawing. There are numerous rulers that your brushes can snap to, including full 3-point perspective. There is even an adjustable mannequin that you can move around to aid in drawing characters in different poses.

Overall, very impressive and quite promising considering how much it can already do while it's currently in beta. I'll be paying close attention to JugiPaint as it develops, and I can already see myself using it regularly just because of how nicely everything already works.