Gimp
From wikinotes
Gimp is the defacto opensource painting program. It's pretty good, but there are things that I miss from photoshop. There are things Gimp can do (like selection masks) that are much more advanced than is possible in photoshop. I still feel like as a photoshop replacement, Krita will be best once it implements macros and a scripting language.
Hotkeys
P Paths (Pen Tool) B PaintBrush E Eraser L Free Select Tool G Blend (Gradient) V Move Tool i Colour Picker Tool Ctrl + D Select None C Crop = Zoom In Ctrl + Backspace Fill with BG colour Alt + Backspace Fill with FG colour Ctrl + U Hue-Saturation Ctrl + L Levels Ctrl + Shift + i (Select> ) Invert Ctrl + i (Drawable>) Invert
Special Notes
There are no Adjustment layers in gimp.
Layer Masks can be applied by right clicking on the layer. They are treated the same as in photoshop.