Google has posted a page with info on some ways to support disaster relief in Haiti, including a couple of very simple/easy ways to donate money.
(There are a bunch of other sites that have posted lists of organizations to donate to as well, but some of those sites seem to be posting any organization they hear about. Be wary of donating money to organizations that haven't been vetted or vouched for by someone you trust; there are presumably a lot of scams out there, trying to capitalize on people's desire to help.)
Janet L points out that not all of the hospitals listed on that Google page are operational. She notes that Hôpital Albert Schweitzer, about forty miles from Port-au-Prince, is operational and is helping patients brought in from Port-au-Prince. (I also heard this hospital mentioned on NPR, I think.)
Someone (sorry, I forget who) linked to an interesting page of "agencies responding to the crisis and accepting donations." I'm unfamiliar with most of the agencies listed there, but the ones I know about do good work. Various friends have also pointed to various religious aid organizations that they support, some of which are listed on that page.
I heard Bill Clinton on NPR earlier saying that the most important need right now is money for water, food, and medical supplies.
A couple of other people on the radio also made the point that although certainly a great deal of aid is needed right now, it'll also be important (as always with big disasters) to provide sustained aid in the future, once this immediate crisis slips out of the headlines.
Interesting article today said that the Red Cross had raised $1.2 million in text-message donations, but added:
It typically takes about 90 days from the time someone makes a donation until the charity receives the money, [...] although the company [that processes the donations] is working with wireless carriers to reduce that delay.
So if you want to be sure your donations will be effective immediately, might be best to make them by credit card rather than by texting. But again, money will still be needed three months from now, so it's certainly not a waste to do the texting approach, and that's probably the simplest way to donate. (And I'm guessing that everyone involved will work to shorten that 90-day lead time for the current situation anyway.)