As usual, Beate wrote a great tutorial that shows you step by step how to make them. This cute little notecard fits inside. I think they'd be super cute for an invitation, or a Christmas card if your distribution list isn't too huge! (While not hard to do, it does involve a lot of scoring and cutting, more than I'd be willing to do for mass production...) They're just really sweet when you want to send something special. Check the postage before you mail them though, as mine if feeling a little heavy!
Please check out Beate's tutorial and the gallery at Splitcoast for all the other people that have joined in on the technique today. Both are listed as links above!