internal Wavebird receivers. The instructions at MaxConsole are pretty easy (if you're into this sorta thing). Basic ingredients include: a Gamecube memory card; a triwing screwdriver; soldering iron; wire; and lots of electrical tape (see pic).
There, now that that's done, we've got 251 blocks of storage for Gamecube saves. Wait, what's this SD slot here for again?
[Via Nintendo Wii Fanboy]