If you are able and willing, you can donate to the hurricane Katrina relief fund through the links below. If you know of any other charities, please add them to the comments below.

Craig's List has links and phone numbers for charities other than the Red Cross (like the United Methodist Committee on Relief that Amy mentions below) if you wish to do so.
http://www.craigslist.org/about/help/katrina_aid.html

American Red Cross donations:
http://www.redcross.org/donate/donate.html

To make a donation by phone, call 1-800-HELP-NOW (435-7669). Spanish speakers can call 1-800-257-7575.

Find out where to donate blood here:
http://www.bloodsource.org/

If you'd like to know how you can help locally, here is the Sacramento Sierra Red Cross chapter:
http://sacsierraredcross.org/main/home.asp?topic=HME&lngKey=en

If you can spare a room and offer free housing to hurricane evacuees, please go here:
http://www.hurricanehousing.org/

Habitat for Humanity will be rebuilding many homes in the Gulf Coast area http://www.habitat.org/