First one, it's Mortal Kombat 2 rom for Sega Genesis, but hacked. It allows you to play with the hidden characters and bosses, and it includes practice and endurance mode, new combo system, run, brutalities, etc. It's called "Mortal Kombat 2 Unlimited". It's created by cool guy Smoke. Here some pics:

Second one, it's a Ultimate Mortal Kombat 3 hack, for Sega Genesis too, called "Ultimate Mortal Kombat 3 Trilogy". It's very cool, it includes all the chars from previous MK, I think it haves some new stages, and a lot of new fatalities for every char. I can't renember who created it though... Here the roster, and a pic:

I too know a similar one, for Genesis too, called "Mortal Kombat Revelations", which gets out some MK1 chars, and add Quan Chi and Hydro Cyborg. I don't own it anymore though.

And the last one, which uses our cool MUGEN engine, it's the "Mortal Kombat Project". Altrough it misses some chars from the Trilogy, it doesn't matter, cos there are new chars around. Examples like Reiko, Reptile Cyborg, Sub Zero from MKDA, Blaze, Hornbuckle, and my favourite one: Chameleon Cyborg. Also it has a cool intro, and 150+ stages (from there I took most of the stages I posted like Portal, the Sky Battle ones, Devil May Cry Tower, The Forgotten City, etc). The fight system was completely adapted to the MK style, even including Stage Fatalities (LOL the things we can do with char-stage interaction). It was created by Prist, from MugenFury. I think actual version is 4.7. Here the roster, and some pics:

Hope you liked them, and good luck if you are going to search them. I will post the links here if I find them again (they are not that hidden though...).