I noticed a few days ago that the Red Cross website was running really slow. (It took me about 5 tries just to get the home page to load.) Well, Amazon.com has a Red Cross donation page, and they’re already set up to handle massive amounts of traffic. (Thanks to the Syndic8 mailing list for the link.) [Edit: the page is no longer active]
There’s also FEMA’s list of organizations offering assistance.