My husband has gotten very into hammocks, and has made numerous types for backpacking and backyard use. He sleeps in them overnight when we are apart or when he is camping -- he's even used them while camping in the snow (with a good sleepingbag and some extra insulation to the hammock itself).
I feel uncomfortable sleeping in a hammock at night, as I normally move a lot in my sleep and I'm afraid that I will fall off in the middle of the night, and I have trouble getting comfortable when I'm side-sleeping because of the dip. So it's certainly not for everyone, but for those who enjoy it, it's really a great option.
When I moved to my first apartment, I was super-broke. I spent and entire $18 furnishing the place, if that gives you any idea at all. I had no bed and no mattress, but I did have a sewing kit. I took a cotton comforter and a thin cotton blanket and sandwiched them around a whole bunch of cheap cotton t-shirts (I took donations from friends for a few weeks). Some thrift stores will give you their tshirt "seconds" (too ugly or torn to sell) for free if you ask nicely. I quilted the blanket side, but not the comforter side, so I wouldn't be sleeping directly on top of ridges. The whole thing took me about a week, working on it whenever I wasn't at my part-time job, and I ended up with a poor man's all-cotton futon big enough for a married couple. We just used it directly on top of a floor. It was firmer than a regular mattress, and a bit lumpy for the first week or so until the fibers and air pockets settled down. My back felt AMAZING sleeping on top of it, much better than anything else I've used since, although my husband wanted to get rid of it once we were able to afford something more conventional. It was very similar to a Japanese style futon that my dad had used for many years when I was growing up. And you can roll it up and put it in a closet during the day if you want to use the space for something else, which is really nice. If I was to make it again, I would probably use a stiffer fabric for the outside layer, and probably cut off the sleeves from the t-shirts before layering them together, just to make things a bit more uniform and easier to work with. I might also quilt several groups of smaller layers together, instead of quilting through the whole entire mess; that was a bit much. I imagine that that would add some more stability to the structure.
I do also like the idea of sleeping on top of blankets piled together. This has the obvious advantage of being easily washable, although it would take longer to set up each night before sleeping. I've done this while visiting family for up to a week at a time, though I haven't tried it for extended periods of time.
One thing to note, we do live in a warmer climate, and I imagine that for colder climates, insulation from the floor would be an important factor. I'm also not really worried about flammability in my bedding, as I almost never use a fireplace or heater at night. In cooler climates, this could obviously be more of a concern.