If you want me to add your site, just send an e-mail to RandX.

Nintendo's Official Site - Go to http://www.nintendogamecube.com for more information about current and upcoming GameCube games.
Game Screenshots - Contains lots of screenshots for just about every GameCube game.
IGN.com - Includes previews, reviews, and media for GameCube games.
Game FAQs - Great place to go if you need help with a certain game.
HonestGamer.com - Very similar to GameFAQs, except it is much newer.
GameCube 1st! - A well designed site with some interesting and original content.
GameCube Central - One of the few sites that is still actively updated. It's updated rather diligently and there is a significant amount of content.

Flying Omelette's Kitchen - A multi-console site with great reviews.
X-Entertainment - A site with well written and interesting articles on video games, TV shows, movies, etc.
The Ham-Ham Club House - A Hamtaro site with a lot of content. I'm also a staffer here under the name Oxnard. Be sure to check it out if you are a fan of Hamtaro.
Czarland Comics - Not a video game site, but the comics look interesting.

Flying Omelette's Kitchen Forums - It's a multi console forum, but there is a forum specifically for Next Generation Nintendo, which includes the N64 and GameCube.
Czarland's Comics Message Board - A message board with forums for video games, movies, music, etc.