Sumo Paint
Sumo Paint has fast become my favourite tool for digital painting. It's again all online so no download required. You just log in and launch the sumo paint app. It has layers, layer effects, blending modes, loads of brushes and loads of effects for the brushes. You can control opacity via a slider as well as brush size and flow rate.
I have found this software very useful and easy to use. If i had to pick a problem i would say that it needs some better tutorials.