I'm a big fan of the internet. It brings me things that I would never find through normal (see legitimate) means. My favorite piece of the internet is currently to be found at http://supermariobroscrossover.com/
Some mad genius took the very first Super Mario Bros and inserted other classic NES era characters into the game. Samus, Mega Man, Link, Simon Belmont and more are all playable. Each of the characters behaves just like they do in their own games. Link has his sword. Samus has her blaster. It seems like it shouldn't work but it does. Brilliantly. Watch the video and you'll get the idea.
If you're a fan of old school NES games (or video games in general [or mad genius creativity]) then do yourself a favor and check it out. Do it quick before somebody gets sued and this game disappears forever.