While many vital services are still in action and exempt from shutdown, many others have been frozen, and could inadvertently have a pretty poor effect on the housing market, especially if this shutdown lasts for more than a couple days.

USDA loans - the USDA loan program is frozen. Since files are approved by USDA offices directly, there will be no more guarantees or housing loans issued by the USDA offices until the shutdown is over. This could have a pretty terrible effect for those who live in rural areas or need 100% financing.