If your household is anything like mine you get too much mail. For us it's a mix of bills, credit card offers, and random stuff from people who sell our names. Once we move this year I plan on using one of the services that stops all your junk mail and plants trees. Until then I'm using the 8 zillion envelopes to my advantage.
When we open mail things go into one of 3 places. Things we need to file go into a pile to be dealt with later. Anything with sensitive data that we don't need to hang on to goes into the shredder. Non sensitive papers go into a recycling bag beside the shredder (to extend the life of the shredder). 80% of what goes into the recycling bag is envelopes that I fish out as needed to:
Keeping receipts- When I travel for work I have to turn in receipts if I want to be reimbursed for food and gas. Two trips ago I learned that pockets and take-out bags are not safe places for receipts. I'm missing once receipt, and of course it's for the most expensive dinner of the trip. No I keep a junk mail envelope in my bag and slip all of the receipts in as soon as I get them.
Grocery shopping-I've done this 4 times now and so far it works well. I write my list on the back of the envelope and put the coupons I want to use inside. More than once I've gone and and then left the store coupon in hand because I got distracted. It also saves me from using other paper.
Writing notes- This is more Gameboy's thing, but envelopes always seem to be handier than real paper when he needs to write a phone number or other random information.
Envelope Budgeting- For those of you who break you monthly budget into envelopes with cash this could keep you from buying (or maybe borrowing from work) new envelopes when you need them.
For receipts (and probably cash) I use the return envelopes that come with bills and credit card offers. Outer and inner envelopes do the trick for grocery's and note writing.
EDITED TO ADD: Two of the services I referred to that will reduce junk mail are Green Dimes and 41 Pounds.