Sadly, my microwave doesn't get cleaned very often. Confession: I don't think I've cleaned it since we moved in (July 2010) until today! It was funk nasty, but it didn't occur to me to take a before picture. Just trust me. It was bad.
So, how did I go from nasty to this in less than 3 minutes, with no chemicals or scrubbing?
Answer: I filled a mug 3/4 with plain tap water and nuked it for 5 minutes. The mess wipes right out, in under 3 minutes (obviously the 3 minutes doesn't include the steam time).
Thanks to the random sister at church years ago who taught me this trick. Wish I could remember who you are right now!