This is crazy cool looking! It’s a free online app you use in your browser. It even works on iPad, and to an extent on iPhone (can’t zoom). You can also purchase access to more advanced brushes and tools, but all the important basics are free.
It supports layers, opacity, filters, and even WACOM tablets. Beta testers have already done some impressive work with it and I’m sure there’ll be a huge rush of new paintings now that it’s public.
I’ll have to play around with it and see if I can do anything cool. The only downside I see is that the canvas is limited to 900px × 446px.