What do Duke Nukem, B.J. Blazkowicz, and Doomguy have in common? They're all stars of a pretty sweet-looking Doom mod, that's what.
As you can see in the above trailer, the aim of Samsara is to take locales and/or characters from the FPS games of old — Duke3D, Heretic, Hexen, Marathon, Chex Quest, Quake, Wolfenstein 3D, and of course, Doom — and mash them up to create an insane crossover where you pick a character, and either fight monsters co-operatively, or tear each other to bits in fast-paced deathmatches. You can also play levels solo, as shown in this video demonstrating the Ranger from Quake:
The mod's been in development for over a year, and is still a work-in-progress. The latest version, for use with the modern Doom ports ZDoom and Zandronum, can be nabbed from the official forum thread over here. Oh, and if you were wondering, yes, that is Jon St. John speaking at the start.