It's been quite a while since XBox Live and PS2 Online brought fun multiplayer to those systems. In the meantime, the Nintendo GameCube has 2 hard to find adaptors, and only one game - Phantasy Star Online.
While Amazon and other retailers list the *existence* of the broadband and modem adaptor for the GameCube, it is nearly impossible to actually get your hands on one. And if you do indeed get a cable modem or phone-line modem adaptor for your gamecube, there is only one game that can be played - Phantasy Star Online.
If you do want to play Phantasy Star Online, you have to pay a monthly charge of $8.95 to attach to the Sega server that maintains the game.
Hopefully the GameCube developers will catch up with the XBox and PS2 soon, and start offering a wider variety of gaming options online for the GameCube. With the wealth of "party games" like Mario Party, the GameCube should be a natural for online, fun gaming!
GameCube Broadband Adaptor