I love decorative napkins. I think they add a lot to a table setting and they can really help to set a theme for a party. Well, I finally figured out what to do with those left over napkins that never get used -- use them on cards of course.
Find an image on the napkin that you like and cut it out. Then peel the (usually two) layers of backing papers off. This leaves you with a single(very thin) layer with the image you want on it. Lightly spray the back of the image with a spray adhesive and mount it onto card stock. This will give you a sturdy piece to work with, and you will be able to place it on your cards and tags.