If a microwave oven is still in working condition, the best thing to do is to give it a second life. A working microwave oven can be given 
away to friends or family or donated to a charity.
A working microwave oven can be left in the front yard with a free sign on it. Usually a microwave oven with a free sign will get picked up by a person passing by within a short period of time. It is best to include on the sign that the microwave is in working condition, to assure that it will be picked up.
Giving a microwave oven a second life serves two purposes. It keeps waste out of the trash dump and saves another person the money of buying a new microwave oven.
If a microwave oven is not in working condition, it can be recycled. Call your local trash disposal company to find out if they recycle microwave ovens. If they do not, ask them if they know a place that does. If your local trash disposal company can not help you, go to Earth911.com. Earth911.com has a search engine that allows you to search for microwave recycling centers in your area. You can also search for recycling centers for various other items in the search engine.
If you are unable to find a local recycling center through your local trash disposal company or through Earth911.com, there are a couple other choices that you still have. Some places take broken microwave ovens and either repair them or salvage parts from them and recycle the leftover parts.
Check with local home appliance repair shops to see if they take broken microwave ovens. Sometimes, home appliance repair shops will repair broken microwaves and resell them, or if unrepairable they will salvage parts out of the broken microwave and recycle the leftover parts.
Another option for broken microwave ovens, is to donate it to Goodwill Industries. Check with your local Goodwill about their policies regarding broken microwave ovens. Many of the Goodwill stores give broken microwave ovens a second life by repairing them and offering them for sale in their stores.