You can donate to Doctors Without Borders here. Quoted from the Yarn Harlot's blog:
The MSF Hospital has sustained damage that means it isn't functioning as a hospital right now. Staff have moved to the courtyard and set up tents and what materials they could retrieve from the building and are doing their best to help people as they can. Doctors who were providing maternity care are now running a trauma centre.You can also donate to the Red Cross which has pledged $1 Million to Haiti relief here. The easiest way to donate is to text the word 'Haiti' to '90999' and $10 will be billed on your next cell phone bill by the Red Cross.
They, and their sister offices in other countries spent all night figuring out who could go and how to get them there, and staff is packing as we read this to get there as fast as they can. They'll be taking inflatable surgery suites with them so they can use that instead of their damaged buildings.
There are directed donation sites set up for Haiti on all of their websites, but I'd like to take a moment to ask you to please make your donation "undirected". Money given to a specific fund can only be used for that area and making sure your donation is undirected helps them a lot. It lets them keep serving other crises that continue to happen while the world watches Haiti, and more importantly, it allows them to be first responders.
On their websites (Canada here, USA here, Other countries can find their site here) when you give there is a drop-down menu that you'll see called "I would like my gift to be directed to..." and you'll see a lot of choices. Please consider selecting "Greatest Needs" or "Emergency Relief" so that they can have the flexibility that they need to do their best work.
The US State Department website links to this list of many different (valid) charities which are directing funds to Haiti.