If you've got a few minutes to spare, you might want to give Johan Peitz's A Super Mario Summary a spin. Whipped up in 48 hours for the 23rd Ludum Dare game development event this past weekend, it's an abbreviated version of Mario's first "Super" adventure. All 32 levels from the NES original were recreated as single-screen challenges, and your goal is to collect all the coins and reach the top of of the flagpole for a three-star ranking.
Condensing an entire level into a single screen is quite remarkable, as it requires a clever arrangement of that level's obstacles. The stages have a unique platforming layout compared to their NES counterparts, but enough of the flavor of their respective levels has been retained. And because all the action is confined to single rooms, the challenge is a lot more concentrated.
There are a few bugs, of course, but that's the nature of having such a short dev cycle. They don't hurt the game overall, so go play and be merry!
A Super Mario Summary [Johan Peitz]