You've all probably seen the pictures of the destruction on television or on news websites everywhere. There is ample coverage. NASA is also covering Katrina - in a way only NASA can cover a large-scale event such as this - from space.
This is one of the before and after pictures on the site. You can clearly see large sections of New Orleans completely flooded. Click the image to go to the site and see more pictures of the area. It's a very informative and fascinating look at Katrina as well as lessons on how hurricanes happen. Go check it out.
I also urge you to find a means of contributing to the relief effort if you can. There are a number of organizations that need everything from cash to volunteers to blood. Check locally or online for ways to help.