I hacked my brother's Wii this weekend. It was fairly painless (and pointless at present). All you need is a copy of Zelda: Twilight Princess (which I don't have, hence using my brother's Wii for this) and an SD card. You use the Twilight Hack as a Zelda saved game, which then causes a buffer overrun when you try to talk to the first character in the game. This is exploited to load a boot loader which loads and runs a boot.elf file off an SD card in the Wii's memory card slot. Pretty easy to do.

There's not a lot out there at present. Wiibrew seems to be the main hub of activity. I tried the GameCube Linux Proof of Concept Linux port; once they add USB support it might actually be useful as that'll open up mass storage and input via a keyboard. There's a Sega Genesis emulator on Wiibrew which isn't too bad, but I found it a bit temperamental at reading ROM images.

Still, it's a start and it's pretty nifty that you can do this without having to modify your Wii in any way. I expect more groovy homebrew will appear as the hardware of the Wii gets better mapped out.