LetsSayThanks.com is a website that allows you to fill out a card and have it sent to a soldier. These cards are completely free and it will literally only take you 10 seconds.
After selecting your card design, you will be asked only for your name (as much as you want to divulge) and hometown. Then you can choose from various pre-written messages for a soldier or create a custom message of your own. You cannot choose which soldier gets the card, but it will likely make someone’s day.
(Thanks A Thrifty Mom!)