Others say...

"No need to get this anymore..."
The only reason people used to get this was for Phantasy Star Online but the server went down. There was also a private server for a while where you didn't have to do the whole monthly fee thing and you could play with people on pc but. Nobody plays on gamecube on that server anymore because Phantasy Star Online: Blue Burst is compatible on that private server now. [...].

"Game Cube Broadband Adapter - EZ & Fun"
I bought the Gamecube broadband adapter in order to play Mario Kart DoubleDash in LAN mode. The unit was very easy to setup, just clipped it into the bottom of the Gamecube. With that I connected it to a second Gamecube using a crossover cable. The cable I used was the Xbox link cable, and it worked perfectly. Alternatively, you can use regular network cables and just connect the two Gamecubes to a router. It will accomplish the same thing. Once you start Mario Kart DD, you'll notice a LAN mode option not available prior to installing the Broadband adapter. Once you have both units connected, select LAN mode and let the two Gamecubes try to detect each other. Mine detected the other Game after the number 198 flashed on the screen (number of seconds I guess), so it took a while. LAN mode was a blast, hope you all enjoy it!

"GCN Modem Adapter Gamecube"
Now not many people know that the Gamecube is Online Compatible.Now If you Hve A High-Speed DSL or Cable Internet Connection you should purchase the Broadband Adapter.Now the Online Compatible Games are Phantasy Star Online Episodes I&II,Phantasy Star Online Episodes I&II Plus,and Phantasy Star Online Episode III C.A.R.D. Revolution.You can Also creat a LAN(Local Area Network) Connection which by you simply connect up to eight Gamecube systems and you play each other.Now the LAN Compatible Games are Mario Kart:Double Dash,1080 Avalance,And Kirby Air Ride.Those Games all of those games above except the phantasy star games will have the Broadband Symbol on the back of the game case.Theirs also a Online Gamepaly program that you can download called Warppipe where you can connect over a LAN Online which connects you to other Gamecubes far away but its really not an online compatible Game The Warppipe site for downloading this setup is [...].
Now if you have a Simple Dial-Up Internet Account connection if you can find which is very rare in store get the Modem Adapter.Now the Only Compatible Games are THE Three Phantasy Star Games that I mention above.Now they say That AOL(America OnLine)is not compatible with Phantasy Star Online but AOL Has Pogo.com letting you play games through AOL Plus I called them myself so don't really listen to that.I found the Gamecube Modem Adapter for $9.00 brand new package but it had dust on it.The Bottom line is you should buy the Gamecube Modem and Broadband Adapter.