I tried this today and was very surprised by just how well it works! So I'm sharing.
To clean a microwave
Put juice of one lemon and 1/2 cup water in microwave proof bowl, uncovered. Heat on high for 3 mins then let stand for five minutes.
Then just wipe down with hot damp cloth.
I had to rub a little bit with the cloth and rinse it out a few times but my microwave definitely needed a good clean and this got 95% of the gunk off with no evil chemicals! Apparently the acid from the lemons loosens the gunk/grease. Worked as well as any of the chemical cleaners I've tried for the same purpose.