Amazon has a page for the Katrina relief effort by the Red Cross. It is a good way to donate to the Red Cross. I like the Amazon page as it shows totals collected.
The story of Katrina is overshadowing worsening conditions in Darfur. Right now the refuges are returning home, but there is no food waiting for them (farmers were not able to plant during the rainy season). Right now, the food shortage in Dafur is probably the largest international humanitarian crisis.