I use mostly MyPaint and Krita, both compiled from source (git). I made a few tests with Corel Painter and Photoshop on another machine but are way to expensive for me and also are not acting quite as expected with the Bamboo tablet.
Both are quite impressive applications, i love MyPaint. With Krita is another story, is still in development and sometimes it glitches, but the cool thing is that has many brush engines. Can use Gimp brushes, MyPaint brushes and many others engines and they might include Photoshop brushes soon. There are also brush presets made by David Revoy - a great libre-artist. So i recommend them both, especially is you use a Linux distro KDE based (needed for Krita). Also work both on Windows, there are some compiled installers but i do not like them as much ... are old somehow and are missing some of the new features