Katrina

If you wish to donate, I recommend America's Second Harvest [Link] first, as they are out of Chicago, and 98% of the funds collected go towards relief programs. [Give.org Report] The Red Cross [Link] would be next, as 91% of donations go to relief programs. (Frankly, however, I'm not impressed with their handling of money from 9/11 and the Tsunami. But, that being said..)
If you want info from someone who is inside New Orleans, check out the Outpost Crystal LJ [Link]
Give blood.
GiveLife.org [Red Cross]
Lifesource
America's Blood Centers