The International Committee of the Red Cross has a Family Links webpage where families can register and look for names. (Available in English and French.) ICRC has more information on searching for relatives here.
The State Department Operations Center has set up the following phone number for Americans seeking information about family members in Haiti: 1-888-407-4747 (due to heavy volume, some callers may receive a recording). You can also send an email to the State Department. Please be aware that communications within Haiti are very difficult at this time.
Google’s crisis response team has an online “Person Finder” accepting and (publicly) providing information about missing persons. Ushahidi, the disaster mapping group, is gathering information here on missing persons to add to the Google database. Also, Ushahidi invites individuals in Haiti to text 4636 (International:447624802524) on Digicel to report emergencies and missing persons.
Occasionally, the numbers or names of missing and found individuals are published. As they become available, they will be mentioned here.