The International Committee of the Red Cross (ICRC) has setup a website to allow people to seek information about relatives who may be affected by the hurricane. The website allows you to register your address so that you can be contacted by others. It may also help you to locate people who may be affected by the hurricane.

However, note that the ICRC has no means of verifying the information sent through the network and thus cannot accept responsibility for any inaccurate information made available on the website.

You can find more details at http://www.familylinks.icrc.org/katrina.
 
Thanks to Melvin.