I’ve seen a lot of cool security envelope lining projects in my blog reading, but I think this stationery set from Patricia Zapata of A Little Hut might just be my favorite. The creative ways in which she repurposes the entire envelope (check out that bookmark from the clear envelop window!) are just brilliant. See her blog for the PDF with instructions for each of the projects pictured.
Tiny Security Envelope House
Security Envelope Pendants
Security Pattern Business Card Envelopes