Magnets: Get some magnet tape or a flexible magnet square (available at craft and hobby stores). Find a decorative shape on a Christmas card that you like. Now cut it out, adding about one-quarter inch to all sides. Glue your shape onto the magnet, and cut out again, this time following the shape exactly to trim off the excess.
Turn them into postcards! Cut off the back of old holiday cards, draw a vertical line halfway across the width on the back, and you have a postcard with Christmas art on the front and a blank back to write/address on.
Placemats: Cut off the backs of old cards, or cut out shapes or pictures from them that are meaningful to you. Laminate or press between clear contact paper. The laminated placemats can be used in several ways, including under pet dishes for a festive look at the holidays.
Some cards are art! For exceptionally attractive Christmas cards, consider framing them.
Use as a photograph's mat. Like the above, you're framing an attractive Christmas card, except this time place a photo centered on top of it, with the card peeking around all sides as a border.
Bookmark: Cut a card into a bookmark shape; these are great for recipe page-holders. Glue another card back-to-back for a stiffer bookmark. If you're a scrapbooker, use your corner-rounder to, well, round the corners. No pokey bookmarks!