This idea from Martha Stewart is easy and fast. You can display many cards on a garland going up a set of stairs. Alternatively, this would also look great on a mantle. Use tiny decorative clothespins from a craft store to attach the cards.
Make a Christmas Card wreath at Paper Source in one of their classes. You could use a sturdy cardboard wreath frame as a base and simply glue or clothespin the cards to the wreath. You'll probably have to choose your favorite cards to display on this wreath because once it's finished, it's kind of difficult to add things to it.
Better Homes and Gardens has another fantastic card display idea. Slide your Christmas cards into the slots of decorative bells. The cards will stand up straight in the bells. The benefit of using this idea is that you can take your cards out of the bells to store after this holidays. It's reusable from year to year.
Here's another fantastic idea from Martha Stewart. Adorn a mirror with shiny metallic garland and clip your cards on the garland.
How do you display your holiday cards?