Left Outside have compiled a good list of links of aid agencies working in Haiti, and their initial responses to today's earthquake. The Disasters Emergency Committee is "urgently assessing the situation against its criteria for an appeal", so may well have information of a DEC appeal soon.
Another route would be to consider donating to the Brazilian civic society groups which run now good programmes in Haiti, such as Viva Rio and MSF Brazil.
That suggestion comes from humanitarian worker (and highly informed activist-thinker) Conor Foley, who has previously written about the Brazilian leadership of the UN Haiti mission, and how those NGOs have made a strong contribution to that south-south international cooperation, often by applying lessons from their work in the Brazilian favelas, .
There is some interesting background information on how aid agencies respond to earthquakes on the ALNAP network website.