Because there are some people who will take advantage of anything, please consider carefully any appeal for aid that isn’t from a well established charity.
I’m recommending the American Red Cross, because they are well established and they specialize in disaster relief.
You will probably want to specify a donation for Hurricane relief, but don’t forget the Red Cross also facilitates communication between members of our military and their families. There will certainly be a lot of soldiers wondering about their loved ones in the devastated areas.
If you you prefer an alternative to the Red Cross, please consider Mercy Corps.
More information is available on efforts in support of aid to Katrina’s victims by bloggers here and here.
Instapundit has a number of links.
N Z Bear is maintaining a Katrina relief aggregator page.
And prayers couldn’t hurt either.