De-makes (2D or retro-style conversions of more recent games) are nothing terribly new, but adding elements of your favorite current-gen games to a classic platformer can turn up similarly awesome results.
Stabyourself.net is currently working on a Super Mario Bros / Portal crossover, which they're calling Mari0. The project currently supports four-player simultaneous co-op. We've embedded two videos below - one showing off early two-player testing, and the second showing a feed of the Giant Bomb staff taking a crack at four-player co-op.