Wrathamon said:The only games that supported proper online play were the Phantasy Star Online series. And this was a subscription based service (with 30 day trial or something) run by Sega.
Other games (such as Mario Kart Double Dash) would support local LAN play only.
For both of these, you would need the Gamecube Broadband Adaptor that plugs in one of the ports on the base of the console.