This website was launched yesterday.
It's purpose is to allow people to post their location so people can know they are okay and to allow people to post that they are looking for someone.
It is international. It is hosted by the red cross.

Two ways to reach it:
1) go to www.redcross.org and click the link from the main page: Red Cross Movement Family Links registry

2) go to this url directly: http://www.familylinks.icrc.org/katrina

There is also a phone number to do the same posted at this location.