GBA/GCN Link Cable
With this handy cable, you can form a connection from your GBA link port to a GameCube controller port, allowing for loads of interactive possibilities between certain games. Many games that have been released on both GBA and GCN platforms utilize this unique link by making certain areas or options accessible only when the two games interact. The GBA has also been used as a separate screen to display private information in multiplayer games, such as Zelda: Four Swords Adventures. Some have seen this unique and innovative link-up as nothing more than a gimmick, but if you own a GBA, as most GameCube players do, and at $10 per cable, what's the big fuss?
RETAIL: $10

Modem Adapter
Instead of supplying stock GameCubes with expensive modems that only a few players would actually make use of, Nintendo decided to provide this adapter for use in the high-speed add-on port underneath the system. Released alongside Sega's Phantasy Star Online, the modem (and its broadband cousin) made a laughably miniscule change in the way Nintendo games are played. Fewer GameCube games have online features than you've got fingers on your right hand. Bottom line: forget this modem adapter ever existed.
RETAIL: $35

Broadband Adapter
Here's the option for those with a broadband internet connection (i.e. cable modem, DSL). Because of a serious lack of online products to make use of these adapters, the Broadband's most important function plays out in LAN ("Local Area Network") gaming. Connect up to four GameCubes to a network hub using standard broadband cables and a few of these bad boys, and you've got yourself a 16-player funfest on your hands. Sadly, only a few more GCN games are LAN-compatible than those that are online. Mario Kart: Double Dash is the most notable LAN game.
RETAIL: $35

LCD Monitor
This product was shown briefly at E3 in 2002, but has disappeared since. Unlike third party devices, Nintendo's screen would have made use of the sharp digital A/V port, which is good. Apparently, two people with GameCubes could have linked the screens together, keeping some information from each opponent. Stay tuned to see if this device is included in a "last hurrah" package before or around the release of the Revolution.
RETAIL: (cancelled?)