What can I do with the glass plate turntable from my microwave oven?

Usually those are kind of heavy, but I suppose you could use it to serve food on for parties.

Or, it could be recycled.

Another thing you can do is see if someone needs one by listing it for free on Craigslist or Freecycle - I just got rid of food processor parts that way.

I've used one as a cake plate before and also for putting the meat from the grill on it with foil over the meat to make the trip in the house and keep warm.

Recycle it, use it as a serving plate or as a spare if/when you get another microwave oven

I used the one from our last microwave as a water-catcher under a houseplant. That way when one of us overwaters it, it doesn't mar the wood, the turntable catches it.

Wham-O one time Frisbee at the garbage dump