Put it out at the curb, in just a couple of days it will be gone. Just remember to call "No Backsies!"
We have a couple of repair shops in our area that will let you drop off a mower for their graveyard. Occasionally they will need to find a part from an old mower that may be out there.
Use slownsteady's idea, and make friends at that shop by 'donating' your neighbor's junked mowers to them too (if they want them). Then when you need help, they remember what you did for them.