Think twice when you walk by an old antique bed at a flea market. If it has a frame that can be rehabilitated and a box spring in reasonable shape, I think you could find some use for it.
via Lettered Cottage
via Cottage Elements
I’m sneaking that last one in, it’s not exactly repurposed but the idea is still good. Come across thin mattresses? Re-cover and stack them together – it’s an out of the box idea for an old mattress frame.