From the Red Cross newsroom:
(1/13/2010) You can text “HAITI” to 90999 to donate $10 to American Red Cross relief for Haiti.
I've done this. You get a confirmation text along with links to more information. It's very well executed, and it's extremely convenient.
Of course, you can also donate by conventional channels:
There are many ways you can support the American Red Cross.
Help people affected by disaster like the recent earthquakes and floods by donating to the American Red Cross Disaster Relief Fund. On those rare occasions when donations exceed Red Cross expenses for a specific disaster, contributions are used to prepare for and serve victims of other disasters. Your gift enables the Red Cross to provide shelter, food, emotional support and other assistance to victims of all disaster. Call 1-800-RED-CROSS (1-800-733-2767) or 1-800-257-7575 (Spanish). Contributions to the Disaster Relief Fund may be sent to your local American Red Cross chapter, or to American Red Cross, P.O. Box 37243, Washington, D.C., 20013. Internet users can make a secure online donation by visiting www.redcross.org.
You can help the victims of countless crises around the world each year by making a financial gift to the American Red Cross International Response Fund, which will provide immediate relief and long-term support through supplies, technical assistance and other support to those in need. Donations to the International Response Fund can be sent to the American Red Cross, P.O. Box 37243, Washington, D.C. 20013 or made by phone at 1-800-REDCROSS or 1-800-257-7575 (Spanish) or online at www.redcross.org.
For volunteer opportunities, contact your local chapter.
Display and share Online Tools to help spread the word about Red Cross Relief.
There are also the secular organizations Partners In Health and YeleHaiti. Here's a good SSA article on those and others.
Update: Jen over at The Atheist Experience has written a nice in-depth article on secular charities.