Sonic 2 Delta

Sonic 2 Delta is a groundbreaking series of ROM hacks by famed Sonic scene member Esrael Neto, which restores several deleted levels and prototype elements to Sonic the Hedgehog 2 for the Sega Mega Drive / Genesis. Various features of Sonic 2 Delta, common across the series despite the existence of several versions/branches of the hack, include the use of art and music from the betas and Sonic the Hedgehog and re-implementation of badniks that had been removed from the final game. The level order of Sonic 2 Delta is also similar to the original intended order, rather than that used in the final version.

Levels:

The order of levels is directly as shown in the newly implemented Data Select menu:

• Green Hill Zone: Straight from Sonic 1!
• Emerald Hill Zone: Contains prototype layout elements, and the Coconut badniks are replaced with the scrapped Snail badniks.
• Winter Hill Zone: The graphics that are used are unlike any descriptions ever given of the winter level, other than the winter setting. There are trees present but not Christmas trees. Also, no graphics from the (also scrapped) desert level, Dust Hill Zone, are reused; this differs from the official team's plans for it to be largely a palette-swap (similar to what Hill Top Zone is of Emerald Hill Zone). The BGM and boss used are those of Marble Zone from Sonic 1.
• Wood Zone: The graphics are similar to what is seen in the Simon Wai prototype. It uses a different BGM (2 player Emerald Hill Zone, but slower). It reuses the ARZ Boss.
• Dust Hill Zone: Uses graphics similar to the mockup screenshot of Dust Hill Zone that was featured in many magazines. It's a desert-only setting. The BGM used is from Oil Ocean Zone but slower. The boss of Emerald Hill Zone is reused for this level, with no alterations.
• Chemical Plant Zone: Small minor alterations were made, such as the loops having slanted corners, like in the prototype.
• Labyrinth Zone: From Sonic 1.
• Neo Green Hill Zone: Sonic 2's ARZ with its prototype name, and music.
• Star Light Zone: From Sonic 1.
• Casino Night Zone: As mentioned previously, it has been reverted to its beta form.
• Marble Zone: From Sonic 1.
• Hill Top Zone
• Spring Yard Zone: From Sonic 1.
• Mystic Cave Zone
• Hidden Palace Zone: The graphics and layout are the same as those found in the Nick Arcade prototype. The BGM used is the same that was used in the final version of Sonic 2. A lift can be found on the long ramp; this was unused in the Nick Arcade prototype. The previous zone's boss is repeated in this zone.
• Oil Ocean Zone - 'beta style', with different music and the ride-able balls found in the betas.
• Metropolis Zone
• Genocide City Zone: The background graphics of Oil Ocean Zone are reused but in a night-time setting. The foreground also reuses graphics from Chemical Plant Zone. Despite having 2 acts, there is no act marker on the title card. Water can be found in Act 2. The BGM is different from that which existed in the Nick Arcade prototype (the track here is the main boss theme from Sonic 3). The Chemical Plant Zone boss is reused here.
• Scrap Brain Zone: From Sonic 1.
• Final Zone: Also from Sonic 1.
• Sky Fortress Zone
• Sky Chase Zone
• Death Egg Zone: Slightly different in that: A. You get 3 rings to start the zone, and B. The music is from the beta.

Source: http://info.sonicretro.org/Sonic_2_Delta