ROM hacking

ROM hacking is the process of modifying a video game ROM image to alter the game's graphics, dialogue, levels, gameplay, or other elements. This is usually done by technically inclined video game fans to breathe new life into a cherished old game, as a creative outlet, making minor edits to change details or to make essentially new unofficial games using the old game's engine.

ROM hacking is generally accomplished through use of a hex editor (a program for editing non-textual data) and various specialized tools such as tile editors, and game-specific tools which are generally used for editing levels, items, and the like, although more advanced tools such as assemblers and debuggers are occasionally used. Once ready, they are usually distributed on the Internet for others to play on an emulator. There is such a big number of hacks available on the Internet, that searching for Mega Man hacks on YouTube will easily reveal at least a dozen.

Mega Man Ultra
Mega Man Ultra is a ROM hack for the game Mega Man 2 that was developed between 2005-2006 and released on 2006 as an addition to the game. The game features new enemies, music remixes from different games, more difficult levels and different named Special Weapons, although the Robot Masters are still the same, only with a palette swap and different strategies.

Story
''20 years have passed, since the capture of Dr. Wily. During that time period, Dr. Light has been secretly creating a new battle robot called X. Six months ago, Wily escaped, and has stolen the blueprints to create X. Will Megaman find the truth behind Dr. Light and X...''

Notable Differences

 * All the Robot Masters are palette swapped. They also have far different strategies than the second game, making their battles much more difficult:
 * Metal Man now throws far more Metal Blades (Saw Cutters) when he jumps, and when he goes to the other side of the screen.
 * Air Man shoots the Air Shooter (Top Spinner) faster and does a small hop before jumping twice to the other side of the screen.
 * Bubble Man now shoots more Bubble Leads (Ice Bubbles).
 * Quick Man shoots his Quick Boomerangs (Ultra Beams) faster and dashes twice as fast and twice the times.
 * Crash Man won't stop shooting Crash Bombs (Vector Claws) if he is attacked.
 * Flash Man now runs twice as fast and when he uses the Time Stopper, he shoots more shots than he does in the original game.
 * Heat Man now charges more energy before shooting himself at Mega Man, and shoots twice as fast. After that, he may get stuck in a wall, trying to shoot flame blobs but is unable to do so, making him very vulnerable to attacks.
 * Wood Man shoots his Leaf Shield faster, it deals more damage, and he jumps to the other side of the screen directly, giving almost no time to be attacked but only in the air.
 * Dr. Wily's Great Temple bosses also have different strategies, and others are also replaced by other bosses, except for Mecha Dragon, Guts-Dozer and the Wily Machine No. 2:
 * Mecha Dragon is now weak to the Bubble Lead (Ice Bubble) and it is the only weapon capable of harming it.
 * The Picopico-kun is replaced by Metroids, and interestingly enough, they are also weak to the same weapon the original boss is weak to: Bubble Lead (Ice Bubble). This is a reference to the original game, where Metroids are very weak to the Ice Beam.
 * Guts Tank is now weak to the Leaf Shield and now shoots Crash Bombs and Ultra Beams. However, it deals minor contact damage.
 * There are now 10 Boobeam Traps, and they now shoot at intervals. They are also now weak to the Quick Boomerang (Ultra Beam).
 * The Alien holograph is replaced by a Mega Man that then transforms into some kind of a jet powered Mega Man that shoots supercharged electrical spheres that spin and now deal slightly more damage.
 * Many enemies are ripped from other Mega Man games and programmed to do what the normal enemies do, the only exception being Wood Man, who has the Super Mario Bros. baddies as the enemies on his stage. The other enemies are also back but others are also palette swapped.
 * The position of the bonuses and the levels is very different.

Bosses
Robot Masters

Wily's Great Temple Bosses

Trivia

 * It is implied that Mega Man kills Dr. Wily in the game, as when the game is cleared, a robot (maybe X) is piloting the Alien holograph, and Wily does not appear anymore in the game.
 * Some enemies come from other games.

Rockman 4 Minus Infinity
Rockman 4 Minus Infinity, Rockman 4 MI for short, is a ROM hack of Mega Man 4 made by Japanese ROM hacker Puresabe. Like Mega Man Ultra, it is a redesign of Mega Man 4, changing the weapon functions, stage design, and boss patterns.

Weapon changes

 * Dive Missile has been changed to the Water Cutter, which shoots a quick stream of water that can be fired in 8 directions.
 * Drill Bomb has been changed to the Drill Torpedo, which fires 3 drills. One shoots forward, while the others crawl along the floor and ceiling.
 * Dust Crusher has been changed to the Recycle Inhaler, which vacuums enemies, pickups, and even robot masters within a short radius. Inhaling specific things can grant you permanent upgrades.
 * Flash Stopper has been changed to the Spark Manbow. Holding B will hold a lightbulb in front of you, which you can use as a shield, melee weapon, or release it as a projectile. Pressing Up + B will stop time.
 * Pharaoh Shot has been changed to the Pharaoh Shotgun. It's nearly identical, but as the name implies, you can shoot 4 projectiles at once, and yes, they can be charged.
 * Rain Flush has been changed to •Toad. It turns most enemies on-screen and even Toad Man himself into small frogs that can be stepped on. Note: Don't step on Toad Man if you want bonus points at the end.
 * Ring Boomerang has been changed to the Holdable Control Ring Boomerang. Not much has changed, other than being able to stop it midair by holding B and being able to shred minibosses with it.
 * Skull Barrier has been changed to the Hell Wheel. It works as a shield against physical and projectile attacks like before, but now it can make Megaman move much faster, and it's much, much more durable.
 * The Balloon has been upgraded to be permanent as long as it's on the screen, and can be moved in any direction. It functions similarly to Rush Jet in Mega Man 3.
 * The Wire has been upgraded to be fired in 8 directions, and can cling to walls. It can also be used as a weapon, but it's generally advised not to.
 * Rush Jet and Rush Coil are replaced with Rush Search, behaving similarly to (albeit more reliable than) it's appearance Mega Man 7, Rush Cannon, a weapon that allows you to literally fire Rush at your enemies for massive damage, and Wish Star, a screen nuke that kills every enemy on-screen.

Notable Differences

 * The Jumbig takes a lot more of damage to kill, and occasionally it launches very fast towards Mega Man, pushing him back once he is hit. However, he leaves out a lot of goods when he is defeated. Starting in the Cossack Stages, he is armed with missiles.
 * You can't select Rush Jet, however it does appear at scripted segments in various stages.
 * In Pharaoh Man's stages, Mega Man becomes cursed. Depending on the curse, he'll fire rapidly, change weapons, run rapidly, even reverse the player's controls. This can be nullified with Toad Man's weapon.
 * Half of the Robot Masters enter berserk mode once their health is low. This gives them more speed and extra damage to the player. The music also becomes faster paced.
 * You can now use Energy containers as storage tanks which do not disappear after you use them and replenish every time you pick up additional health when the main bar is already full. However there are only 4 containers in the game now (instead of 6 + 4 sometimes dropped by Eddie)
 * The Boss Rush stage in Wily 3 is a lot different than other Megaman games. You now have to browse through 8 different environments within the same level in order to find the Bosses, and the teleport capsules appear there to give Megaman quick access to different environments.
 * There's a secret final boss and stage after you beat the Wily Machine. To do this, the player must activate the Wish Star when Wily Machine's health is only 1 bar left. (This also shows the true ending after you defeat the "true" final boss).
 * Inhaling Eddie in the game with the Recycle Inhaler (Dust) will make him disappear for the rest of the game and he'll be replaced by a grey Eddie.
 * The stage themes are taken from a lot of games past to present, such as Kirby 64, Mario Kart, ActRaiser, Gradius, Adventure Island, and even obscure Japan-exclusive games like Hokkaido Serial Murders: Okhotsk Vanishes.
 * Some enemies from Megaman 7-10 appeared in their 8-bit style form for the stages.
 * Some Robot Masters return in this game. Shadow Man is a reoccurring miniboss fought in various stages. Wave Man is an optional boss fought in Dive Man's stage, and Crash Man is an optional boss fought in Drill Man's stage. Beating them gives access to the Balloon and Wire, respectively. Others appear in the form of Chimerabots, robot masters with some abilities taken from other robot masters. (Cutman with the ability to throw three Rolling Cutters akin to Elecman, Wood Man with Quick Man's agility, and Needle Man being able to clone himself and ricochet his needles in a similar fashion to Gemini Man)
 * Many enemies and bosses are re-modeled in this game, like the Yellow Devil MK-II and Copy Megaman.
 * A lot of Engrish!

References:
 * The Mega Man Network
 * Destructoid