Operation Hero Miles is, I believe, a great way to help US soldiers who are allowed to come home for the holidays, but have to pay for the flights themselves (!) come home. You can donate frequent flyer miles. Participating airlines are AirTran, Alaska, American, Continental, Delta, Frontier, Midwest, Northwest, Pan Am, Southwest, United, US Airways. You can donate online, over the phone, or via e-mail.
Many soldiers on R&R leave must purchase high-priced last minute airfares to connect to their hometowns once their military flights land at the airports in Baltimore, Dallas or Atlanta. Soldiers on "Emergency Leave" must still pay the full cost of their domestic travel when they are rushed home for the family death, birth, illness or other emergency. The military does not pay the cost of their travel in the United States, so your frequent flyer miles are a huge help.
I just donated the 10,000 or so miles that have been sitting around unused in my United Airlines account. Somebody can sure use them. (I have lost track of some other accounts on Delta or American ... I wish I knew if they had any miles in them!)
What a simple way to make somebody else's life better.