Lindsey over at The Postman's Knock creates the cutest illustrations and prints for sale at her Etsy shop, but every once in a while, she puts up a freebie up on her blog, like this witchy envelope template just in time for Halloween. Just print, cut, and glue. Easy peasy.
You can download the template right here, and check out all her freebies here. While you're at it, you should look at the rest of her blog; it's pretty fantastic.