You're probably thinking about the usual area under the staircase - like the one that Harry Potter lived in which is created by the angle of the staircase itself. Well, how about a completely new idea that will have you smacking your head for not thinking of it?
There's usually lots of room for storage underneath a staircase, but what about the space within the staircase itself?
Most people will climb stairs an entire lifetime without noticing that the steps looks like drawers. Like a gigantic bank of drawers. Drawers perfect for storing junk.
Well, this idea has not been lost on some clever Australian inventors. They hacked a staircase so that some of the steps can be pulled-out to expose a standard drawer type storage area. Yeah, I didn't think of it either.
I don't know if you can "buy them" yet, as each installation will be fairly custom, but it appears to me that the staircase drawers would be a pretty simple do-it-yourself task or an afternoon project for a skilled carpenter/cabinet maker.