Apple has posted a nifty tip for Mail users who miss the discontinued iCards feature from .Mac: You can use Mail stationery to create an attractive card-like message with your own photos.
For Leopard users, Mail comes with 30 email templates you can customize in lieu of sending an iCard. Personally, I think an email is better than an iCard anyway, what with all the greeting card spam that goes on.
You can use photos that you take with Photo Booth, for example, or pictures from iPhoto. Customize it with a clever note, and away you go.