FreelanceSwitch has a great list of ideas for ways to thank your clients during the holidays. The suggestions range from charitable gifts ("buying" a sheep through Oxfam) to creating custom T-shirts from CafePress. Of course, the fallback gift of food can be a big hit, as long as you stay away from fruitcake.
Everyone at Freelancers Union fondly remembers the year one of our consultants sent us a chocolate babka from Zabar's.
How about you? Do you make it a habit to send holiday gifts or cards to your clients? What do you send--and do you think it makes a difference for your business?