A PC version of Super Mario Bros. with one twist - the whole game is backwards. You start from the flag at the end of the last level (though I wish they implemented the defeating Bowser and rescuing Peach cutscene backwards) and do everything backwards - put coins back where they were, stomp already-flattened Goombas to revive them, etc. It even shows the credits at the beginning. It's tricky because you have to not touch a coin after collecting it backwards, not stomp the same Goomba backwards more than once, not fall farther than you can normally jump, and so on, but it's entertaining all the same.
(If you don't get it, the subject line of this post is a backwards lyric to Mario Twins by Group X)
EDIT: NIP!! Megs and nukes say Nair a-- Flush now so rare purples say air puffs, Ned? See it? No? NARF!!...Yep! Your fish lay hang it note! "Yeah, well, f*** on the easy SNAAARRRR!!" says the open Malfoy! Oh, ow! Yep! Oh, no! (Translation: I beat it! I was disappointed it only had three levels though. Hey, I have an idea: quote backwards Group X again!)