Slightly off topic: my most useful extension cord is a little orange 6 or 8 footer I picked up somewhere. It's perfect for running the dremel tool, soldiering iron, etc. There are a lot of times when you want a little extra cord, and don't need the whole 25-foot coil laying on the ground.
Back on topic: I've got several extension cords out in the shop, between 12 and 25 feet. I just figured out a good way to store them: I hand coil them into decent sized loops, maybe as big as the distance from my waste to my knee. I've got pegboard on my walls, and these coiled cords hang very nicely in the pegboard hooks that are a half-circle. (these are the good, quality, tight-fitting pegboard hooks, not the cheap metal crappy ones) I have three of those hooks on the wall, "stacked" one above the other with one peg hole empty between them. The three extension cords are overlapped on the wall, starting from the bottom up--all easy to grab, easy to grab the one I need (12-foot or 25-foot), easy to put away, and don't take up a ton of space.
-Brad