In anticipation of this problem this year, I decided to do a little research and I was able to find some fun and easy ways to reuse Christmas cards.
Since I like to crochet, I liked this idea from favecrafts.com. Make a crochet border around old Christmas cards in order to make Christmas ornaments or to decorate photo albums.
NoBiggie.net shows how to make Christmas tags out of old Christmas cards.
There are many great ideas on eHow.com including, donating them to the St. Jude's Ranch Recycled Card Program, using old cards as postcards, using several old cards to make your own card, making placemats, and saving them for crafts to do with children.