Well, it's July, but I bought this a couple weeks back, in June.
Gamecube: Final Fantasy Crystal Chronicles.
I've been wanting to try this game out for awhile. It is suppose to kind of be like the old game gauntlet, although more of an RPG, but the big draw for Nintendo was to try to use the GBA as a controller for the game, when played in multiplayer.
There were a lot of games that had some type of usage of the GBA link-up cable. Some, like Phantasy Star Online, used it for mini games. Some games used it for extra items, or to unlock extra locations. One of the things Nintendo really tried to sell the system as was for football games, where you could secretly select plays on your gba where none of the other players could see it. It's not something I've used much personally, but here is a link to compatible games:GBA/GC Link-up Compatible Games