Play This Thing reviews Megamash, a weird chimeric Flash game that combines several kinds of play and requires players to figure out how to use the mechanics of each genre to solve puzzles:
At first, it seems like a simple platformer in which you play a bunny collecting carrots and avoiding enemies. But half-way through the level, you pass through a barrier into a sidescrolling shmup, and become a spaceship shooting aliens with the space bar.
In otherwords, the game is actually a sort of mashup of seven different games, and you pass from one to the other over the course of a level. And indeed -- this is the clever part -- you often need to do something in one "game" in order to advance in the level. Thus, in the rabbit platformer, a crate you need is surrounded by impenetrable blocks; but when you fire across the shmup/platformer boundary, the bullets turn into falling fireballs, which obliterate impenetrable blocks, freeing the crate you need.