Many years - though not necessarily every year - I use some of the Christmas cards we receive to make a Christmas card ball. I've done this so many times now that I don't need any instructions, but if you want to make one, you need 20 circles and some fast drying glue. These instructions on the web are very good at explaining the process. I chose the 20 cards I wanted to use - I usually pick just the ones I like the look of, but sometimes I make sure to include a special one - for instance, the angel on the yellow background was drawn by the daughter of a friend, and the Christmas tree on a funky map was hand-made for me by a friend (Hi, Ruth!), so both of those definitely had to be in. It's also nice to have fairly flat cards, otherwise it's hard to fold them.
After cutting the circles, I use a triangle template and a ruler and something (this year, a little screwdriver) to score some lines to make the folding easier.
Then you begin to assemble the parts - 5 each for top and bottom and a ring of 10 for the middle. The most important thing is quick drying glue as it means you can really assemble these easily, without paperclips to hold things together while they dry, etc.
And here we have it - a finished Christmas ball. This year, I even remembered to write 2014 on it somewhere - I usually forget the year! I think I have about 7 or 8 of these now, in various sizes - they are a great way to remember some of the cards you've received and a lovely Christmassy decoration, without spending any money...
The rest of the Christmas cards get turned into gift tags or bases to build ATCs on - and the offcuts go into the recycling box - very little waste at all.