Modifies most of the IOS installed in your Wii so you can play games through the normal disc channel and play gamecube games if you install the extras.
There are other ways of doing this and not everyone likes it, but I've had no problems.
There has been a lot of work done on updating and improving the cIOS. When the USB Loader started to get popular, Waninkoko had been releasing cIOS36rev_(version). cIOS36 is a customized IOS based on system IOS36. It installs as IOS249. Subsequently, most revisions and updates (despite what version of official Nintendo IOS they are based on) will install as IOS249. IOS249 is the IOS that enables most of the hardware hacking such as USB Hard Drive Loading and DVDX. When new games come through with additional patches/IOS requirements, the coders will release a new cIOS to be sure to include all of the patches that are included with that update. That is why suddenly we are looking at cIOS38_rev14 instead of cIOS36_rev13. For maximum compatibility, I would suggest getting the latest stable cIOS release. What constitutes stable? Well...you'll need to do your homework and read forums/release notes.
sociallyantisocial for IOS 249 data