Good answers here already. Lenders today are expecting 10%-20%. With poor credit, they'd probably require 20%.
As the other answers also note, it also depends on the size of the mortgage you'd qualify for. You could put 20% down, but if you'd only qualify for a $80,000 mortgage, you still wouldn't be able to buy.
On the other hand, if you're buying creatively, "a good downpayment" is as small as possible...assuming you can cover the monthly payments. With a lease-option, a subject to, or a number of other strategies, a few hundred dollars or less. Not everyone will take such a small downpayment, but some sellers out there will. (With a lease option, it's not even a downpayment; it's an option fee. And with a subject to, you're actually buying the house for your $10 or $100, subject to the existing financing.) I've done lease-options on property with virtually no option fee.
Hope that helps.