"From Cologne with Love" By Are We Designer. A design firm in Germany created these retro custom envelopes. It perfectly encapsulates their creativity and represents their brand well (Found on Behance)
Ivy Hotel Sales Kit. Ivy is an upscale hotel in San Diego, and they created an appropriately sophisticated envelope to help better sell their banquet halls and meeting rooms. (via CRU Agency)
"Newspaper Boy" by Alex Kwan. Alex is a designer who created a package showcasing his skills. The envelopes are so simple, yet so much fun!
Flywheel Design by Nicole Kraieski. There's something about a black envelope that is so eye catching and unexpected. Plus with the beautiful splash of color in the upper left corner, these envelopes really pop. (via http://www.behance.net/nicolekraieski)
Dog Envelope by Michael Hacker. It doesn't get much color than that! Michael has done an amazing job of using all the space available to create a truly special envelope (via creativereview.co.uk)