I bought the Humanitarian's Hachimaki and edited all my class load-outs for it. The Hachimaki is $7.49, and the all of the proceeds from that, plus two other hats, one costing about $20, the other $100, will go towards the Red Cross's Japan Earthquake/Tsunami Relief Fund. The bummer about this is that Steam isn't setup to distribute donation receipts for your taxes, but if getting a tax benefit is something you want, you can always donate to the Red Cross directly.
http://www.teamfortress.com/post.php?id=5170&p=http://www.pcgamer.com/2011/03/24/team-fortress-2-selling-100-20-7-hats-for-japan-aid/The hats will be gone from the TF2 store on April 6. They will likely never be included in the random drop item pool, so this is your only chance to get them.