I know that people have been saving the varied papers from the insides of envelopes for quite some time and a while back I posted about a photo book I had made from old envelopes. I still have a large‘collection’ and continue to find them useful.
This is a sketch of the view from my window and one interpreted in ink and stitch using envelope paper.
Ro Bruhn has done some lovely paintings using her old envelopes, take a look at her post for 30th September and she may inspire you. This is one of mine on a journal page with a monoprint in the window made on magazine paper.
'Hens Teeth' on Flickr has done some of my favourite monoprints using the cellophane windows in envelopes.
Recently I found Carrie Holder's blog; she does some interesting experimental work and this is a link to her blog where she shows some of her work using envelopes. I’m sure you have come across loads of different ways of using envelopes on blogs or Flickr. Leave a link in your comments and others can go and have a look.
Here is a link suggested by Jackie the Prepaid Envelope Art Group on Flickr.