Portaledge: a device that functions as a "portable ledge", for sleeping, resting, or belaying. Typically made of nylon fabric and aluminum tubing, with adjustable nylon webbing to level the platform. They can be single (at left) for one person, or double (at right) for two. Note that the double is divided such that one person's head is in one direction, the other person's head in the other. They make for very comfortable, but cramped, sleeping. Several examples are below. The portaledge comes in very handy when it is 11 pm, no natural ledge is in sight, and you are feeling exhausted.

For a wonderful review of the history and development of hammocks and ledges, see this thread from SuperTopo.