Bunk beds probably aren't going to solve most of our small space problems. If they would, though, this solution is awesome.
Floating off of the wall, cantilevered bunk beds leave the space below and between beds completely open. The result is a clean, unencumbered look that would work really well in a small space. A similar design, in image 3, pairs a cantilevered bed above and cantilevered desk below. Could be perfect for small guest rooms.
Images: via Ohdeedoh, via Daddy Types, Nathaniel Grey