If you have a microwave that's covered with spattered sauce and popcorn butter, you can kill the odour and loosen up the gunk with a bowl filled with bicarb soda and water.
All you need to do is put two tablespoons of bicarb soda in a microwave safe bowl and add a cup of water on top of that. Then put the bowl in the microwave and run for three minutes. The bicarb soda will kill the funky smells and the water will start to steam clean the spots and stains. Give it a wipe down afterward and your microwave is good as new.