Here are over a dozen ideas for making your own envelopes or adding something a little extra special to them (with decorative paper, doilies, fabric, etc.). Great inspiration here for holiday cards, wedding invitations, special announcements, whatever you like. Nothing too complicated in this bunch and the expense is quite minimal, have fun!
Made From Heart-Shaped Paper : A heart shape is cut from decorative paper then cleverly folded into an envelope.
Collage Style : Made with brown kraft paper and decorated with bits of scraps such as old receipts, parking tickets, etc.
Lace Paper Envelope : This one’s made with a 9″ square paper doily and lined with vellum.
Inter-Office Style : Another project made with fabric, this one closes with string wrapped around two buttons (at the flap).
DIY Templates : Here’s how to take any envelope you like, take it apart and make your own cardboard template.
Decorative Paper Liners : Can be made for any size envelope, shows you how to measure for a perfect fit.
Pretty Flower Pattern : Bright and cheery with a green background and floral print, folds into a square with space on the front to note details.
Teeny Patterned Template : Includes decals and labels on the sheet (via pdf download).
Hello Design : Includes template and liners with colorful stripes and the word “Hello” (green, blue, orange and pink).
Bonus: Microsoft Office templates (for Word, Publisher, etc.) can be found here , you’ll find dozens of freebies in assorted sizes (business, coin size and more).
Also see the motherload of ideas here: Over 100 Templates & Tutorials  (melstampz.blogspot.com).