Storage for strange orphans you're guilted into raising in your home?
More seriously, we used to have a little walk-in closet under the stairs in the house I grew up in. My parents put in a bunch of crates on either side, leaving a path through the center, and added a string for the pull light that was long enough for a 4-year-old to reach it. I kept all of my toys in there, and even ended up using it quite extensively as play space when I was a child. Since it was out of the way, I didn't get in a ton of trouble the time I 'decorated' the walls with my crayons.