@MissCourt - I never would have thought of that. I own a spirit level just because my wife love to drive me crazy by tilting the picture frames. It's sort of a game of ours -- so I cheated by buying a level.
It really comes in handy when when we rearrange things in the living room. We've on a second story and the floor boards are uneven in some parts. So whenever we move something, even a little bit, we check it to make sure that my art pens won't go rolling off of the top.