It’s hard to believe you can play a game this awesome for free, in your browser. Programmer Jay Pavlina spent over a year recreating the entire Super Mario Bros. game, with playable characters from other NES-era classics.
I just powered through World 1-1 as Mega Man, shooting Goombas with my arm cannon, and machine-gunned the hell out of World 1-2 as Bill from Contra. Also available: Samus Aran from Metroid, Simon Belmont from Castlevania, and Link from the Legend of Zelda.