Super Mario Mods
Video Games Mashed
Here are a couple of related posts so I thought I'd give you a two for one deal.
Using the Doom engine, someone created a mod where your space marine destroys some goombas with a shotgun.
Super Mario Bros. Vs. Doom
Mario takes on the Covenant from the Halo series. The creator has also released the game as a playable download.
Mario Vs The Covenant