Sonic 2 Hacking Guide was the name given to the first ROM hacking documentation released to the internet for a Sonic game. The document was created by saxman in late 1998. It was directly inspired by information released by Tom Sonic detailing how to access Hidden Palace Zone through savestate modifications. A draft was written on 8.5 x 11-inch printer paper and mailed to SSNTails. When saxman got his own computer for Christmas the same year, it was typed up in a text file and submitted to Simon Wai to be offered as a download on his website. When Andy Wolan offered saxman hosting at SSRG, saxman created an HTML version of the guide. The guide broke down many barriers, including being the first source to document how to modify levels, music, various game configuration settings, and demo sequences, among other things.
The guide hasn't been updated since 2004. However, its impact and influence has lived on with many ROM editing tools directly based on information in the Sonic 2 Hacking Guide, including SLEEP, SonED, and Sonic HackingWiz Pro to name a few. A copy can be found on the Hacking CulT server here.