Apple has setup a donation page in iTunes allowing users to donate money to help Haiti earthquake victims via their iTunes account.
Click the Donate button under the amount you wish to donate. 100% of your donation will go to the American Red Cross. Because iTunes is not sharing your personal information with the American Red Cross, the Red Cross is unable to further acknowledge your donation. You will receive an email receipt from the iTunes Store which will serve as your only acknowledgment of your donation. The donation will also appear on your credit card statement as a transaction with the iTunes Store.
Donations can be made in the following amounts: $5, $10, $25, $50, $100, and $200.
Bummer that you can't use money already in your account. I routinely get gift cards and have money in my account, but I can't use it to donate :(
(I've already donated via SMS, but was looking to give this, which is essentially free money).