The USB Gecko is a Wii/Gamecube development and hacking tool which connects to the USB port of the computer, and to the GameCube memory slot of the Wii. It can be used to upload homebrew, to use your computer as a remote terminal under Wii Linux and as a remote debugging tool. For more information see the USB Gecko homepage. It has recently been discontinued and the schematics released on Google code.
To code computer side usbgecko application you need :
The USB Gecko is a two step converter to convert EXI<->serial<->usb. It is a fancy serial port. All the fun is provided by software. No, the EXI interrupt is not possible to trigger from the PC.