There are an increasing amount of Katrina-related donation scams out there. Please, please educate yourself before giving. If you are not already familiar with the organization to which you choose to give, a certain amount of due diligence should be done.
Check ScamBusters as a starting point. The Better Business Bureau can only be trusted so far as companies pay for their listing. FEMA offers a list of suggestions, but follow their advice at your own risk. FEMA hasn’t had a great track record so far in the wake of Katrina, as their actions seem to have hurt more than helped. Also be aware of any connection between a charity and a religious organization, as some charities make these connections difficult to discover without research.