Wells Fargo has a new program that starts at a 500 credit score. The down payment amount is tied to the credit score, so the lower the score plan on a larger down payment. Check out the details at wellsfargo.com.
I am a Mortgage Broker not a bank but, I deal with several lenders that will loan to borrowers with scores below 620, I have one lender that will go to 580. The down payments for these lenders is the standard 3.5% FHA requirement. I would be more than happy to assist you with this process if you would be interested.