There are two different $100 down payment programs that buyers might be able to take utilize:
Good Neighbor Next Door
First is the Good Neighbor Next Door program, searchable on HUDHomestore.com but limited in applicability. See more information on this option here.
FHA Financing
Most likely what you are seeing reference to is an FHA financing program for HUD Homes for FHA buyers. You can learn more through our HUD Resource Center.