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.