No HDD on the Wii eh, Nintendo? Big mistake. Makes Wii owners sad. Never mind, while Nintendo may be lacking the will to make a storage device for the Wii, others are more adventurous.
Matrix have parted the curtains on the WiiZii, a plug-n-play mod (mod! avert your eyes!) that allows you the user to connect a SATA HDD to the console, which when plugged in will let you load homebrew files on the fly. The mod itself also includes 2mbit of internal memory, as well as the promise of future firmware updates.
It's no official solution, but then, do you honestly think there's ever going to be an official solution?