Secret Maryo Chronicles is an open-source 2D sidescroller game that borrows heavily from, you guessed it, Super Mario. From fire-flowers to turtle shells, it's the same gameplay through and through, only with a seemingly endless list of custom levels in addition to an original overworld adventure.
The problem with SMC is that it tries so hard to be a clone when its real strength lies in what it does differently. The graphical cohesion the game might otherwise have is ruined by the incorporation of various mutated forms of visuals from graphically disparate installations in the franchise. It's the same with the music: the game uses some rather nice adaptations of the classic Nintendo themes, but even so I can't be alone in occasionally wanting to chop my ears off when I'm played a looping excerpt from the most oft-repeated video game soundtrack of all time.
The program does have its strengths though. As with many open-source games, the options menu seems to go on forever and there is a lot that can be done with the built-in level editor. The eponymous Maryo can run in addition to his other skills, and the timer ticks up instead of down, making the game experience more friendly to casual players.