In the past, I've just thrown them out. However recently while making a heart quilt for my dd, I simply couldn't waste THAT much material. I made the corners into 1/2 sqaure triangles by sewing another strip before cutting the 'extra' off. Now I have three stacks that are 4" high!
What I'll probably do is sew them into a strip. Then sew it vertically on a backing to a quilt, off center of course! I did something similar with extra 5" blocks that I had left over from a house block quilt I made for our housekeeper at work. As it turns out, I think she likes the backing better than the front. Her favorite color is the main fabric on the back. Go figure!