MoveOn.Org has put together a grassroots movement for people to offer housing to those left homeless by Katrina.
HurricaneHousing.Org is a website where you can volunteer a spare bed to someone who doesn't have one. Housing is most urgently needed within a day's drive of Louisiana. According to the website, over 37,000 people have offered housing so far.
According to the site:
Many shelters actually already have Internet access, but folks without Net access can still make use of the site through case workers and family members. Hurricane victims or relief agencies will contact hosts and together decide if it's a good match and make the necessary travel arrangements. The host's address is not released until a particular match is agreed on. If hosting doesn't work for you, please consider donating to the Red Cross to help with the enormous tasks of rescue and recovery. You can give online at: MoveOn.Org.