Megaman X3 is one of my favorite games in the X series. However, one thing I didn't like about the game was how Zero was touted as a playable character, but only to a very limited extent. He could only be called to play a small portion of a level; he automatically switched back to X whenever you reach a boss or mini-boss door, and you can't switch back until you exit the level, meaning you can only use him for 1/3 of any given stage. So he can't fight bosses, he can't use Maverick weapons, and he only has one life; die once as Zero and he's benched for the rest of the game.
Well... a rom hack has recently been released to rectify all of that. This mod makes Zero FULLY PLAYABLE. Except for the intro stage, you can switch freely between X and Zero, Zero can now play through the whole game, fight bosses, use Maverick weapons, collect hearts and Sub-tanks, utilize Dr. Light's upgrades, etc.
You can download the hack here: http://www.romhacking.net/hacks/888/