The International Federation of the Red Cross reports that over three million people have been affected by the earthquake that struck Tuesday (and it's still much too early to even estimate the death toll). With the country's infrastructure devastated by the disaster, contributing money instead of supplies would be the best bet. Here's how to aid in the relief efforts.
P.S. Here's an interesting Huffington Post article with a roundup of relief efforts currently in motion, country contributions, recent updates and quick facts about Haiti and earthquakes.