An idea that emerged from random conversations: pick the neighborhoods in the U.S. with the highest foreclosure rates, and give buyers a substantial tax credit--say 20-30%--on the purchase price after a fixed period--say five years--in which they hold and maintain the property. You can add features to prevent damaging speculation, like requiring a substantial down payment.
The basic idea is that property in a lot of these neighborhoods is already cheap, but people are afraid to buy for fear that prices will decline another 20% to 30%, and even that the neighborhoods will become dangerous ghost towns as foreclosed residents move out and aren't replaced. Provide a cushion against further decline, buyers will swoop in, prices will stabilize, foreclosures will diminish (and short sales will increase), and pressure on the banking system will be reduced. If politically necessary, you can also add features to prevent windfall profits, like taxing any future profits on these properties as ordinary income.
I understand that the crisis now goes well beyond U.S. domestic mortgages, but surely stabilizing that market couldn't hurt.