You live in a small space and you need a place for occasional guests, but you don't want a sofabed. Or maybe you live in a really small space, a bed takes up too much room, and you don't want a sofabed for yourself. Daybeds are a great solution, but honestly, what adult wants to sleep on a twin-size bed? Take a look at the Lubi Daybed. In the daytime, use it as a daybed/couch, but at night, remove the top, place it on the floor, and you have a double bed.
$749 at CB2. (There's also a Lubi II for sale at Apt. It has different legs, and costs about $50 more.)