Krita
Krita is a complete creative sketching and painting application with advanced, commercial quality features. It's free, open-source and available across all major desktop platforms. User Interface - An intuitive user interface that stays out of your way. The dockers and panels can be moved and customized for your specific workflow. Once you have your setup, you can save it as your own workspace. You can also create your own shortcuts for commonly used tools.
Pop-up Palette - Quickly pick your color and brush by right-clicking on the canvas.
Brush Stabilizers - Have a shaky hand? Add a stabilizer to your brush to smoothen it out.
Brush Engines - Customize your brushes with 9 unique brush engines. Wrap-around mode - It is easy to create seamless textures and patterns now. Press the ‘W’ key while painting to toggle wrap-around mode.
Resource Manager - Import brush and texture packs from other artists to expand your tool set. Layer Management - Krita comes with vector, filter, group, and file layers. Combine, order, and flatten layers to keep organized. Add drop shadows and other simple effects using visual styles. Full Color Management - Krita supports full color management through LCMS for ICC and OpenColor IO for EXR, allowing you to incorporate Krita into your existing color management pipeline.