I'm gonna make it easy for you to choose a reputable charity to donate money for those affected by Hurricane Katrina. The American Red Cross has always been my best bet because they are an internationally recognized organization in existence since the American Civil War, mobilizing aid directly to those in need (not through some second party). Give what you can; $10 won't kill you and it will make you feel good the rest of the day.
Click here for donation options. Easiest thing is to donate right online. Please note that their website is a little slow because everyone is trying to donate online. Give it time and try again later if you don't make it the first time.
[Update 9/1/05] As with the tsunami last December, Amazon is also facilitating donations to the Red Cross so here's another link for you.