http://www.habitat.org/disaster/2005/katrina/I don't usually push my own brand of giving on others or even advertise it much, but HFH really does a lot of good work and they WILL be instrumental in re-building many of the areas that were completely wrecked by Katrina.
I've worked with them over the years and donate my time to them when I can; you get to help people out, meet great people and you can even learn a thing or two about carpentry and home repair.
If donating money isn't your thing and you live nearby, consider spending a day or two doing some good 'ol fashioned work to help out in the near future.