Super Mario Bros: Revenge Of Bowser
Okay, so the title is about as original as Peach getting kidnapped, but this Mario fan game is one of the best we've ever played. There's a sizeable community of people making their own SMB-themed games these days, but the ambition of the person who created this one (s/he goes simply by the name Hello online) really makes it stand out.
It takes the structure of Super Mario 64, Sunshine and Galaxy (central castle hub, stacks of levels inside, multiple tasks in each one to unlock the rest) and transposes it into a full-scale 2D Mario game using the Super Mario All-Stars graphics set. It's kind of a Mario's greatest hits compilation, combining the new games with the enemies and features of Super Mario Bros. 3 and Super Mario World. It even includes the veggie throwing from the western version of SMB 2, making it the fan game for the long-time Nintendo player. Accordingly though, it's also harder than a concrete bastard (Unless our 16-bit skills have floundered in this decadent 3D age), so be ready to mix your joy with a few screams when you first start. Get it here.
And speaking of Bowser...