Outside of dust bunnies and stray socks, it's amazing what you can store under your bed! There are entire storage systems designed to stow beneath your mattress, and depending on the height of your bed that space can mean the difference between having "enough" storage in your home and not. In my time, I have stored everything from linens in rubbermaids to guns under my bed. (Gasp, I know! I'm in Texas, folks, and my fiance has an antique western shotgun...we don't have kids and it's not loaded!) When I was living in 250 square feet in San Francisco, the space was truly prime real estate. I'm not sure I would have been able to keep many of my pieces of artwork were it not for storage beneath the bed.
I'm currently blessed with large closets, and so there's really very little under there. Outside of those dust bunnies and stray socks, that is. I know that as we settle into this home and grow as a family, that space will become more and more valuable.