There are some places that will recycle them.

Donate them to your local Boy Scouts. Many are working on a merit badge and they might be useful. If nothing less; park them at the end of your driveway with a "For Sale' sign at a nominal price, they will be gone by morning.

Donate them to a Goodwill. Don't they refurbish stuff like this as part of a worker training program? Or maybe it's Salvation Army.

Two times I took my mower to have it serviced Larry Johnston because it would not start and it was only 2 years old.  Each time repair cost just under $80! Recently, same thing, I would crank and crank and it would not start.  Went on to YOUTUBE, followed some instructions, bought a $8 gasket and spent 45 minutes taking the carbreator apart and it starts right up!

Garage sale?

Put them on the street and hope they get stolen.

I buy a used mower for $50-$80 when an old one breaks.  A lawn mower shop, pawn shop or handyman may haul away a non-working mower or ask you to drop it off.

I have also repaired mowers following videos on YouTube. Most problems are easy to fix.

Yes, or check with your local lawnmower / small engine repair shop. They like to work on them and resell them for a few extra dollars.

Recycle or donate them.  A

I always try the end of the driveway with free sign. I figure if someome picks them up they are going to be used and not put in a landfill and that is a good thing.  Have done this with things as small as a bathroom trash can and as large as a desk.  It works.

I would leave it by the curb and advertise as free and needs work . Someone will take it.

Sell them!

Criagslist or donate. 

Larry Johnston Donate the mowers to an organization that will recycle them. One time I put a weed whacker on the curb for the trash service to take and it was gone in a matter of minutes.

Bring them to Corpus Christi and give them to my neighbor Diego.  He fixes them and resells them - a good little income to help support his family.  And yes, he puts a sign on them by the curb.

But yes, a sign on the curb that says FREE works!

Toss 'em. Maybe I'm wrong, but I don't know too many people that would prefer to the spend the time and money to fix a mower when they could just buy a new one or a used one that already works.