List of NSMB2 levels with Secret Exits (no spoilers for the exit, just the level they're in)

[TomaTito]

If you know Spanish, I recommend you visit guiasnintendo.com, it's the official Nintendo walkthrough site managed by the Iberian division (also give it some hits for the sake of my cousin who works there :P).

For New Super Mario Bros. 2: http://www.guiasnintendo.com/index.php?option=com_wrapper&Itemid=329