Ashoka, through its Changemakers program, is sponsoring a new competition, How to Provide Affordable Housing. There's still almost three weeks left for entries, but already they have 20 strong ideas, including one from our own Vinay, the hexayurt.
Competitions and prizes are an excellent way to spur innovation in the public interest, and while this one doesn't have the cachet of, say, an open source design initiative, it's still worth checking out.
One way to provide affordable housing is to pay people enough that they can afford a house. Raising the minimum wage isn't a design solution, but it's also not like arriving at a famine with cookbooks.