If you have an abundance of envelopes that you don't use and are considering throwing away, stop. A number of organizations can accept unused stationery, including envelopes. Before disposing of your unused envelopes, consider donating them to a group or individual who might be able to put them to use. You'll be helping somebody out and doing your bit for the environment at the same time.
Visit or call local thrift stores to see if they might be able to sell your excess envelopes. Even if they can only raise a minimal amount from the sale of your envelopes, it will be better than your off-casts being sent to a landfill site. Make sure that the envelopes you're offering are of reasonable quality, as donating poor quality goods to thrift stores can take up the valuable time of the store's workers. If you get any takers, you can deliver the envelopes yourself, or put them in a larger envelope and post them.
Contact a local recycling company. Many businesses around the country recycle paper-based goods and turn them into something that can be used again. Most will be happy to relieve you of your envelopes if you have a large enough quantity; it is, however, unlikely that they'll be interested if the amount you have is relatively small. You'd be better off taking your envelopes to a public recycling point if this is the case. Some recycling companies might not be able to use your envelopes if they have plastic windows, so it's important to mention this early on if yours have. Find out about paper recyclers in your area by visiting the Environmental Protection Agency's website (see Resources).
Select a local worthy cause or non-profit organization that might be able to use your envelopes. Underfunded public services and non-profit groups such as schools, drug education programs or youth clubs and outreach initiatives might be able to find a use for them. These types of organizations need to save on costs wherever they can, so money not spent on things like stationery can be redirected to core services. Locate non-profit groups in your area that might need the envelopes (see Resources).