A warning - FEMA has a list of organizations taking cash donations.  One of the top on the list is 
Operation Blessing.  This is radical cleric Pat Robertson's organization.  I suspect money you send him is more likely to be invested in race horses than given to flood victims.  Or maybe he'd use it to hire a hitman given his comments last week about Hugo Chavez.
The other organizations on the list seem good.  My money went to the Salvation Army.  I don't share their religious beliefs, but they're reliable and honest. When you need them they're there.