Over at DIY weblog IKEA Hacker, they show us how to take an existing, traditional bookshelf, but move it to an unused space—high on the wall—to save space on the floor. Now, you'll need to climb up a little stepladder of sorts to reach the books, so you can't necessarily have anything else under it, but for this particular hack they put it in their stairwell. It's perfect because you have room to move about the stairs, but can grab the ladder when you need a book, and it gives you more space on the main floors for the things you need.
These particular instructions use parts from IKEA, but you could probably get some similar pieces at your local big box hardware store. You'll need a suspension rail and some wall anchors to do the hanging, and of course a bookshelf of your choice. Hit the link for more detailed instructions, and let us know your floor space saving tips in the comments.
Suspended Besta Bookshelf [IKEA Hacker]