If you're a personal friend of mine, you've most likely heard of this one. For those not in the loop, MUGEN is a flexible fighting game engine, allowing you to create your own fighting game. You can finally pit Street Fighter's Ken and King of Fighters' Terry against each other outside Capcom VS SNK. Wanna get more creative? Try Guilty Gear's Sol versus Fate/Stay's Saber. Even crazier? Mario vs Naruto. How about Peter Griffin vs Ronald Mcdonald, played in space? Yep, you can do it here. Want to make your own character? You can do that, too, with a character editor such as Fighter Factory.
Make it look the way you want to, and pummel a boss character with a broken Dan, if you're that into it. Frankly, some of the best fight videos I watch actually come from the outrageousness that you can cause in MUGEN. Check out this trailer; Not made by Electbyte, the creator of MUGEN, but it does the game justice (as most Japanese renditions of things do).
And finally, a bit of proof of the sheer wackiness the game can create.
You can find download links for MUGEN, in its various styles, as well as links to all the characters, stages, and custom content you want at Mugen-Infantry.