From Sonic Retro
Sonic 2 "Simon Wai" Prototype (Also known as the Sonic 2 Beta)
The first early prototype version of Sonic the Hedgehog 2 to be found, and has been the backbone of this site for years. This highly unfinished, and barely playable game requires a Sega Mega Drive/Genesis emulator to run it. Press A+Start to enter level select. You will need other cheat codes like debug mode to fully explore the zones, so read it up while you're downloading.
Sonic 2 "Nick Arcade" Prototype
After almost 8 years of only having Simon Wai's beta, drx found the community another Sonic the Hedgehog 2 prototype. Though it's not fully confirmed, this build appears to be identical to the version featured in the show Nick Arcade. It's even earlier than Simon Wai's prototype, featuring only 5 playable zones. Press Up, Down, Down, Down, Down, Up, A+Start to enter level select.
The Sonic 2 "Betas"
On February 23rd 2008, drx released six more betas to the community. These are much later prototypes than the Nick Arcade and Simon Wai builds, and cover the last minute changes leading up to the final release. Betas 6-1 and 6-2 are identical except for a few bytes in the header.