(Original Guide by Vladikcomper)
Source: Sonic Retro Thread
Commits:

• NemDec: de69b5d
• KosDec: b3f2ce6
• CompDec: 6d0dddc

A proper write up will be provided in the future, along with notes on additional compression formats, and how these can be used. In the meantime, please consult the linked Retro forum thread.

An optimized Nemesis Decompressor

http://pastebin.com/cAJSJ0np

A much faster Kosinski Decompressor

http://pastebin.com/EtYf1vz6

Comper Decompressor

Decompressor's code (ASM68K version): http://pastebin.com/aMrfKTbz
Decompressor's code (AS version): http://pastebin.com/fVp8d5Fr