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Extra Mario Bros. by ATA | Complete Game Hack
Console Name: NES (Nintendo Entertainment System)
Base Game Required: Super Mario Bros. (non-combo version)
Compatible ROM Name: "Super Mario Bros. (JU) (PRG0) [!]"
Download: http://www.romhacking.net/hacks/369/

Features:

• New maps, new graphics, new enemies, new power-ups, new physics, new final boss.
• It’s inspired by Metroid and in fact, some tiles are imported directly from it.
• It’s practically a new game.

Reviewed by "Mama Robotnik" (via http://tinyurl.com/nackkt8):

This redesign of Super Mario Bros is a marvel. Rather than a level-by-level system, the engine has been redesigned to be more like Metroid, with a series of linked sectors in a large world (which you can backtrack through). Through exploration, Mario earns permanent upgrades such as terrain-destroying fireballs, different types of projectile, different types of boots (so Mario can walk on Lava) keys and double-jumping, allowing him to progress further.

The game also offers multiple endings dependent on percentage of items collected. Should you collect them all, the final boss changes to a huge Bowser (more impressive than it sounds, especially on the NES).

It is worth noting that this is NOT a hack of the Metroid Engine (as would usually be the case with hacks). It is the Mario Engine, re-programmed with an arsenal of new capabilities. This is why I consider it notable. Extra Mario Bros is a tough (but fair) game, which rewards exploration with constant surprises.

Screenshots: Gameplay Video:

That looks pretty awesome, has me tempted to mess with the recent beta firmware just to try it out.

Okay scratch that, not tempted, gonna check out the new beta firmware right now. X)

Kudos

This hack is the truth man.