In the aftermath of the Katrina disaster, we're seeing some innovative uses of online tools to help people recover and to understand what happended. Some of these may be useful examples for organizations working on other policy/social issues, so here is a sample (and please add comments below with other suggestions!):

People finding people
Master listing of people locator services by Red Cross. (9/13)
One of the earliest efforts, the Missing Persons Forum from NOLA.com.
FamilyLinks from the ICRC. (100,000 registered by 9/6/05.)
KatrinaList.Net, a collaborative online effort that developed over the course of a few days into a large scale volunteer effort.
NCMEC (National Center for Missing and Exploited Children) has modified their database services to provide a listing of kids missing because of Katrina.
Medtopia Community, helping people and their physicians to find each other. (9/13)
Katrina Help Finder: geographical-based postings for locating people, equipment, supplies, etc. Uses Microsoft's MapPoint and Google Maps. (9/13)

Finding information about affected areas
What Happended to My Neighborhood? from NOLA.com.
Katrina Information Map is a fascinating application marrying Google maps with collaborative information posting. Article about it on WIRED..

Connecting survivors with helpers
HurricaneHousing, a project of MoveOn.org .
CraigsList - New Orleans housing
Finding opportunities to help: VolunteerMatch (e.g. within 20 miles of my zip code.)

Information for survivors
Katrina Help Wiki, an effort started by people who had worked on Tsunami relief.
A collection of resources by the Federal FirstGov.gov.

Citizen news and photos
Katrina photos on Flickr.

Analysis: what happened?
Lots of voices on this - mainstream media and others:
Wikipedia provides a valuable overview of the development, chronology, and outfall of the storm. Wikipedia had its first entries about katrina on August 26.
There are various timelines from different blog writers, with varying perspectives, no surprise:
Rick Moran/"Rightwing Nuthouse" - and see the many comments, TalkingPointsMemo, ThinkProgress, and the BBC - with related news, photos and video.

Addendum:

Ideas and discussions about mitigating disasters
Refugees International's Joel R. Charny describes valuable lessons learned from its work on international refugee situations.