I think it's just Phantasy Star Online (Both games, I think).
Additionally, there are several "rebels" who have found a way that you can network your Gamecube through your pc, and use their software to trick your Gamecube into thinking it's tied in to a LAN network.
This allows you to play LAN-only games with people online if you prefer those games over Phantasy Star. I've never done it, but I've heard several people discuss it.