It's no secret that the Wii is a pretty awesome game system. To this day, the systems are still sold out at major retailers. Even Amazon is always sold out within hours of getting a shipment in.
So is it possible to make an already fun system even funner? Oh yes, it certainly is. The new aspect of fun comes from the Homebrew Channel. Adding this channel is sort of...unsupported by Nintendo, but it's one of the easiest hacks you'll ever perform.
Gizmodo put together an excellent guide for setting it up on your Wii. You'll need a spare SD card, as well as a copy of Twilight Princess. Other than that, everything you need can be downloaded for free.
One of the best features of the Homebrew Channel is the ability to play retro games from the NES, Super NES, Sega Genesis, even GameBoy. You can download the emulators required to play these games right from the Wii itself. But getting the games themselves requires a bit more work and is, to put it nicely, a slight form of piracy.
But if you feel like continuing, a good site for downloading the ROMS of old games is rom-gods.net. I strongly urge you to only visit this site if you are using Firefox with AdBlock. If not, beware that the site contains some very adults-only advertising.
There are, of course, other ROM download sites out there, if you feel like searching. Once you've downloaded a ROM, you have to put it in the appropriate folder on the SD card, then fire up the required emulator and play!
So, in conclusion, the Homebrew Channel is an excellent addition to any Wii system, but it definitely requires a fair amount of technical knowledge as well as a willingness to download some pirated material.