I have tried Krita, an open source and free to use drawing program for OSX, Windows, and Linux.
- contextual menu for switching the drawing tool.
- customizable color picker at the right
- fullscreen mode
- opacity and size of the brush/tool always available at top
- layers and blending
- opens photoshop documents
The UI made sense to me immediately, and I don't notice any hangups with the software. It does what I expect. I'll be using this. Since I am not an illustrator, you should take my recommendation with a grain of salt. Test this out for yourself. It is free, and works on all major OS's.
The test drawing above was made with a mouse. I ordered a drawing tablet for $60 from Monoprice since my ex has the Wacom I previously used. I wanted the kids to use it at her place.