Haiti

For the godless among us: Mercy Corps is a wholly secular charity that directs the vast majority of its resources to helping people in need. It's very good about ensuring sustainability of its efforts in the community, too.
http://www.mercycorps.org/

Doctors without Borders

Thanks for this.

Not to be crass; but what is needed is money (as opposed to donations of food and blankets, etc.). The major organizations had headquarters in Port au Prince and are struggling to respond. However, Partners In Health www.PIH.org has built and staffed many hospitals in rural Haiti and their facilities and personnel are intact and able to respond. They accept donations.

I just heard on NPR that giving via cell-phone ($10 a pop) has set a record of $4 million. That sounds great; the amount is simply added to your monthly bill. However, it can take 3 months for the $ to reach Haiti. The cell-phone charity is not a good way to meet an emergency.

Joe is correct that money is what is needed right now. The infrastructure -- including the facilities for landing stuff on the island -- is heavily damaged, and in many cases completely destroyed. The best thing to do is send money to those agencies with the resources, knowledge, and contacts to best spend that money on behalf of the Haitian people.

I just donated to a local charity, Feed My Starving Children. Though largely supported by churches, the mission is very straightforward: provide meals designed to treat malnourishment to as many as possible in the poorest parts of the world. The meals are specifically designed to be nourishing, gentle to the system, easy to transport and store, and easy for the recipient to prepare. (Just add boiling water.) They have an established presence in Haiti, which means they have a vehicle for putting that money to work right away. Also, they have a reputation for making the money go a long way. The organization is run primarily by volunteers, and just seventeen cents pays for all the costs to get one meal to the person who needs it (mainly children, but they do also feed adults knowing that the adults can't care for the kids if they're starving themselves).
Feed My Starving Children

There are loads of good charities; this is just the first one that came to mind for me.

Oh, and I have to second anjou's recommendation of Doctor's Without Borders (aka Medecins Sans Frontieres).
Doctors Without Borders

They are well-equipped to move into disaster areas, and also have an established presence in Haiti.

That reminds me of another agency, which has already mobilized in Haiti: Telecoms Sans Frontieres. Their website appears to have crashed, though, perhaps having been overwhelmed. They're working to restore some semblance of telecommunication service, which is absolutely vital for organizing rescue, relief, and rebuilding efforts. Once their site comes back up, here's the link:
Telecoms Sans Frontieres