Riivolution is a homebrew application. It is an "on the fly patcher" that allows you to "modify" files while they are loaded from a disc. To keep the application legal, only original Wii discs are supported ("and thats not a bug" says the developer). Riivolution can be downloaded from the official website. The latest version is 1.04 (release: 16th September 2010).
How to work:
(There is the option to install Riivolution to the Wii menu, provided there is enough space left.) Riivolution creates a folder (named "Riivolution") on your SD-Card. In this folder a new folder named "config" is created. The "config" .xml files (a few information about the disc inserted) are saved here. The "patch" .xml files have to be saved in the "Riivolution" folder (examples can be downloaded on the official website; for the .xml files that are required for the NTSC-US version of Super Mario Galaxy 2 and more, look at our forum). If Riivolution notices, that a file on the disc (the filepath which contains the file is defined in the .xml file) is needed, it reads that file from the SD-Card (instead from the disc). That's the "patching" progess. The good thing at this progress is that the files on the disc are never harmed and it is the fastest way to load and test custom files on a Wii game.
on iPhone, iPad, AppleTV, PC. It's free.