From a site I haven't really visited for a long time (even for a Resident Evil flash parody favorite), Newgrounds.
Super Mario Crossover flash gameHonestly, even though I'm not sure if Newgrounds have lessen the "questionable" ads (like the stuff they have back then), this flash game is too hard to resist sharing.
I mean, come on! You know you want to take out Bowser's goons with the spreader gun from Contra! Okay, maybe that's just me. Maybe you want take out Bowser's goons with Link, Samus, Simon Belmont, or Mega Man for other choices (other than Mario but then again, you might as well pop that Super Mario Bros. cartridge back into your NES if that's where you want to go

). Amazingly, they actually play just like their respective game.
P.S. You might want to grab a gamepad, plug it in and then map the keys to the gamepad via a program like Xpadder. Plays better that way.