The Kombat Pavilion

Mortal Kombat Beats of Rage Games
These have been out there for quite some time.  Beats of Rage is a customizable (or moddable) game engine that is reminiscent of those old school arcade beat-em'-ups such as Double Dragon, Streets of Rage, Final Fight, etc.  What's really great about this is that it's been ported to so many different systems such as Windows, DOS, Linux, DreamCast, PS2, Xbox, PocketPC, PSP, GP32/GP2X, AmigaOS, and MorphOS.  As with anything customizable, sooner or later someone will make a Mortal Kombat version. Check 'em out:

Mortal Kombat Konquest - by Git/\nn0 - a really cool & fun game that really captures the Mortal Kombat feel.  The basis for the game is the relationship between Baraka and Mileena - you can choose to play as either one - and their plan to takeover Outworld for themselves.  It has a high quality air about it, and has tons of details that make it feel like an arcade quality game.  For instance, before you fight Motaro, you can hear him gallop off-screen, as he gets closer and closer.  There are Shang Tsung paintings hanging up in his now-empty throne room.  (Also, in the Windows version, Reptile and Kano are hidden as playable bonus characters.)  Most of the enemies you fight are generic pallete-swaps, with the bosses as key exceptions.

DreamCast & PC - Burning this .cdi file will create a disc that is playable on both your DreamCast and PC (Windows)!
PC only - Just download this zip file if you want to play the PC version on Windows.

Mortal Kombat - by bWWd - another fun game!  This one has less story to it, but a little more variety.  In this game, you can play as a lot more characters.  In this roster we have Scorpion, Raiden, Kitana, Kung Lao, Johnny Cage, and Liu Kang.  Most of the enemies in this game (at least, in the beginning) are custom edited sprites of Shang Tsung's Masked Guards, though later on there are many other monsters such as wolves, dinosaurs, and others.  There are stacks of skulls that remind one of MKSM, and there other references to MK as well scattered throughout the whole game.

DOS - Here's an early version, a demo for DOS.
Windows - Here's the final version, for Windows.

Mortal Kombat Unlimited - by Magggas - another really cool & fun game. Very similar to the one above, but with more levels, more playable characters (such as Sub-Zero), more opponents, more special moves, the inclusion of weapons that you can pick up and fight your enemies with, and more still!  Check out what could be the best version of MK on Beats of Rage yet!

All Versions - This one zip file includes the .PAK file and the OpenBOR files for just about all systems.  Windows users: Move the file "MKU.PAK" into the "PAKS" subfolder under the "WINDOWS" folder.

Mortal Kombat: Outworld Assassins - by Magggas - With the focus on the ever popular ninjas, Scorpion, Noob saibot, and Sub-Zero, the player gets to fight through many more battles in this adventure. 3-player co-op is a wonderful feature!

All Versions - Extract the files to a folder your choice, and be sure to move the .pak into the appropriate subfolder.
Want more information on this program?  Beats of Rage was originally develeoped by the Senile Team, and though it's no longer maintained, distributed or supported, the open source continuation of the project - OpenBOR - lives on.  Check out more information on it here.