Sonic the Hedgehog (North America/Europe) (1991)
This is a rom from the very first production copies of the game. Later versions of the game are said to lack the debug mode cheat, though this is unconfirmed (as are any other adjustments).

Sonic the Hedgehog (Japan) UNCONFIRMED (1991)
The Japanese variation of the game came months after the domestic release and a few changes were made. The differences are minor and just a bit strange; the clouds in Green Hill scroll now, there is parallax in Marble, the underwater segments of Labyrinth have a weird wavy effect, and the level select is, like, in order!

Sonic Special Stages (International) (1994)
When locked on to the Sonic & Knuckles cartridge, the original Sonic seems to behave just like any other cart you could lock on (we'll excuse the animation and color for simplicity's sake, alright?). If you hold all three buttons down, however, you'll enter a bonus game where you face 134,217,728 sphere special stages...if you make it through all of these for whatever reason you go back to the beginning. Argh! The data for this game is stored in S&K, but it's worth noting that the North American and European release of Sonic 1 was used to make this rom.

Mega Games 6-in-1, Volume 2 (Europe)
This is the second volume of the six-in-one Mega Games collection and includes Columns, Revenge of Shinobi, Sega Soccer, Sonic the Hedgehog, Streets of Rage, Super Monaco Grand Prix. Sega Soccer is also known as World Cup Soccer and World Cup Italia 90, but its official name is assumed to be World Championship Soccer.

Mega Games 10-in-1 (Latin America)
The company TecToy (the harbingers of darkness responsible for a 20-in-1 compilation on the Master System) brings Sega products to the Brazilian market and occassionally releases a game of its own exclusive to this territory. This time it's a 10-in-1 compilation that contains Sonic, Super Shinobi, Super Monaco GP, and a freaking insane seven other unknown titles...it's no Super Mega Hong Kong Deluxe 500-in-1 Wonderful Magic Game Pack, but is it really that inconvenient to get up and switch the carts?

Sega Classics 6-Pak (North America)
The Mega Games compilations never made it to American shores except in this release (and the 4-in-1, actually), which is a slightly altered version of the second volume that substitutes the dated Super Monaco Grand Prix with the more gracefully aged Super Hang-On. Sega Soccer is also renamed World Italia 90 here.

Sonic Classics (North America/Europe)
The Classics collection is known as Sonic Compilation in the region of its origin, Europe. It includes Sonic the Hedgehog, Sonic the Hedgehog 2, and Dr. Robotnik's Mean Bean Machine (though wouldn't Spinball have been a better choice?).

Telstar Multicart (Europe) MISSING
Each Telstar Multicart is a set of two games on a single rom, naturally only released in Europe since those Europeans just love sticking roms together. Here Sonic the Hedgehog is coupled with, of all games, James Pond 2: Robocod.