[akeck]

There's a sub-culture of origami related to this called "Envelope and Letter Folding".

[anorakoverflow]

Looking at the links in the Wikipedia article I discovered this wonderfully “old web”-style page with various envelope folding guides[1]. I found it so endearing that I sat down and tried a few of them out, which was great fun.

[1] http://www.orihouse.com/elfa.html