The Wiinja is a first generation Wii modchip. It is made by the Ninja Team.
So far there are 3 versions available:
GBAtemp Wiinja Review
Like the other first generation modchips, the Wiinja is not a console firmware hack but a driver's chipset hack that allow the reading of copied medias.
The Wii has three different driver's chipset since it's released :
The first version of the Wiinja is not compatible with the D2B chipset witch is the last chipset included in the Wii since December 2006 or January 2007.
Since Wiinja V2, all the chipset are accepted.
You can check the Wii chipset version compatibility with the serial number of the console.