I yanked out my old gouache set a couple days ago. I had used them before for mixed media pieces, but had never given them a fair shot as the main medium for a piece. Now that I've gotten a chance to thoroughly play with them, I've found that I like them very much. They look a lot like watercolors, but they have the added benefit of layering very well without diluting the color underneath. Also, the finish is matte, as opposed to the glaze-like finish you get with some watercolor, so they can be easily worked over with other mediums.
melon juice gouache and sharpie on A5 paper click image for full size