Super Mario Doom
There's been a lot of buzz about the Super Mario Doom video... but everyone's question after watching the video is "Where do I get it?!?!" Well, wonder no more, here it is! This is the .wad file for Super Mario Doom. All you'll need to do is dig up your old copy of Doom or Doom II, grab the Super Mario Doom .wad file and start blasting some Yoshis!
To use this .wad, unzip it to your installed Doom or Doom2 directory. Next, drop to a command prompt (RUN -> type 'cmd'). Goto your installed Doom/Doom2 directory and run Doom with the following flag:
doom -f Mariodm.WAD
Oh, and just in case you haven't seen Super Mario Doom in action yet, here's the YouTube video: