Cell Phones for Soldiers is an organization founded by two teens in Massachusetts, dedicated to helping service members stationed overseas stay in contact with the folks back home. Through fund raising activities and money made through recycling old wireless phones, they provide prepaid calling cards so that service men and women can phone home. Each recycled phone buys about an hour of talk time, which is a whole lot more useful than just keeping that old phone in the junk drawer.
There are designated drop off locations for phones, or you can print a postage-paid mailing label on their website. They're a bona fide 501(c)(3) charitable organization, so your donation may be tax deductible to the extent allowed by law (check with your tax advisor for specifics).
They also have a sister site in Canada, with donations there going to help out Canadian soldiers, sailors, and airmen.