These are some envelopes I made once. I picked up an envelope template kit at Lunalux, bought rolls and large sheets of patterned paper everywhere (Paper Source, CorAzoN, PaperGami), and cut out my own envelopes.
The templates were of a variety of styles, from long, slim envelopes you might use at formal parties to accordion-edged mailers that held quite a lot of inserts. One friend in particular, in Madison, WI, received the bulk of my creations. I never found a reliable way to seal them: glue stick tended not to bond well with certain weights of paper, and sometimes my envelopes would fall asunder in the middle of being processed. Sometimes the contents were returned to me, sometimes to the destination, but mostly they were simply lost/destroyed. I would recommend that people start using letterhead: even if your envelope is destroyed in processing, the Post Office can still return your letter to you.
I especially liked this pattern of paper because of how exotic it looked. I tried to efficiently cut one envelope next to another, so as to maximize the usage of this lovely paper, but sometimes I was tempted to place one of the large red designs in a key location on the envelope. I had suitable decorated tissue paper to line the inside, though that was redundant: the paper was impossible to read through as it was. Patterned paper like this also necessitated a nice clear mailing label on the front, of course, a small sacrifice of design for the sake of practicality.