Last night I found one of the greatest levels ever made - it was a tutorial someone had made on how to make a level (and, indeed, how not to).
For example, you start in a very plain area. A speech-bubble at the end says "Don't use plain backgrounds. It looks terrible."
You then pull a switch, and everything vanishes. You're left in a Garden-themed level, with music playing. Another speech bubble a little way along says "Do add music and sounds - it makes the world come alive."
It's a truly fantastic level.