OJR has a wiki with a good list of links on Katrina. Metroblogging New Orleans has some good links too. (Via Trust but Verify)
UPDATE: From comments, here's the I'm OK site, a potentially valuable resource for storm victims and their loved ones.

Another UPDATE:
Meetup.com has created http://hurricane.meetup.com for relief efforts for victims of Hurricane Katrina.The company says that "Meetups have proven to be fundraising machines -- and the objective of Hurricane Meetups is to help people pool resources and raise money. Meetup has waived all fees for anyone organizing Hurricane Meetups."

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