Main Facts
This has every GameCube site you could think of and what it has there.

Official Sites
Nintendo Basic information on GameCube and games.
Nintendo At E3 Has info, lots of screenshots, and downloads for games.

GameCube Sites
Planet GameCube Good site with game listing, previews, reviews, hardware listing, release dates, codes, and more.
GameCube Extreme Not that good of a site. Only has basic info on the system and games. Has some pictures.
Gamecube Network Good site. Has previews, reviews, screenshots, latest news, release dates, and more.
GameCubicle Good site. Has a list of games saying which ones are good and bad, and tells information about them. Has a store too.
IGN Cube Great site. News, Info on Games, Reviews, Previews,
Features, Hardware, Guides, Codes, Release Dates, and Gamestore
GameCube Central Okay site. Includes articles, previews, reviews, release dates, interactive forums, etc.
GameCube XL Okay site. Previews, reviews, screenshots, etc.

Video Games Directory Great site. Has links to all kinds of games. Not just GameCube. Easy to find what you want.>