I’m loving this idea to make security patterned business card envelopes from discarded mail in this tutorial by Jackie at Crafting a Green World.
Jackie writes:

Security envelope papers are captivating the craft world. There are so many different security envelope patterns, and most of us get them with our bills or junk mail so they seem to be in abundant supply. There is even an ongoing project on Flickr called Security Patterns to gather and categorize security envelope patterns from around the world. I took a look through my file drawer and found more than a dozen different security envelopes and instantly began wondering what I could make with them.