Lots of good GIMP tutorials
Contaminated effect tutorial:
Shows various overlay techniques and how to create some nice effects with the pen tool and stock images.
Circle effect tutorial:
How to use layer mode and blend tool to create cool circles
Fancy glow effects tutorial:
How to create glowing lines and fading the end of the lines and also adding glow to objects that are not created from a path.
Transparent Glass tutorial:
A short tutorial of how to make text look like a transparent glass.
Random set of tutorials, mostly GIMP:
Easy animation in GIMP
About digital painting and GIMP, a lot of text at the beginning, but quick and easy tips after halfway:
Set of easy tutorials about how to draw simple background elements:
Colour theory basics with GIMP:
One way of using colours with GIMP:
Pixel art and sprites
Good stuff to read
Ten things every game needs by Mark Rosewater (parts 1 and 2)
Creating good game art when you’re not an artist
Art for non-artists
Diving deeper to art
From the very basics of digital and traditional painting and drawing to more advanced techniques