The obvious is to give money, of course. The American Red Cross is a tried-and-true source for disaster relief. Oxygen is donating $1 for everyone who signs up through their social networking donation site. DonorsChoose is running a very smart campaign, where you donate money to help fund schoolchildren to put together volunteer efforts; there's nothing like teaching the next generation that helping others is a priority, while helping those others at the same time. (Note: the DonorsChoose Katrina campaign is not yet featured on their home page, but I've gotten emails about it and it should be up soon.)
There are plenty more places to give; a search for "Katrina Donations" will turn up a bunch of reputable ones.
For some people, that won't feel like enough. (Hell, donating money sure doesn't feel like much to me.) For those who are wondering what more they can do, Hurricane Housing is trying to match the newly homeless with people willing to donate space in their homes to house them. Particularly if you're in the South or Southeast, this is a great way to make a concrete difference in the lives of those affected by this disaster.
Unfortunately, I can't plausibly host anyone in my very small one-bedroom apartment for any length of time. While I'm going to sign up to donate housing on a very temporary basis, I'm looking for some other way to make a concrete difference short of going down there and offering my time. If people come across other not-just-donate-money resources for Katrina, I'd appreciate it if you could point them my way.