I remember there were quite a few instances where I wanted to paint an overlay over my game using the grain merge blend mode from GIMP. Now, with shaders, it's possible! (And even easy.) Here are a bunch of GIMP's blend modes as shaders. You can cycle through them with left and right arrow keys and increase / decrease alpha of the overlay image with up and down. All are fragment shaders.
N3QN also made bunch of shaders for some Photoshop / GIMP layer / blend modes that are available in his topic Photoshop Style Tools And Blend Mode Shaders. Check them out. If what you're looking for isn't here, he might have it.