Sonic CD

GameFAQs (Sega CD)
Provides information, guides, cheats, reviews, and a message board.
http://www.gamefaqs.com/console/segacd/data/587997.html

Sonic CD (PC)
Information, screenshots, cheats, and a review.
http://www.gamespot.com/pc/action/soniccd/

Wikipedia
An article about the game, with a list of levels.
http://en.wikipedia.org/wiki/Sonic_the_Hedgehog_CD