NIS Downloader (Nintendo IOS System Downloader) is a PC Utility designed to allow easy access to the resources present on Nintendo’s Servers for the Wii console. Using this tool, you can download many system titles into WADs ready to install.
Quick and easy!

This program requires the Microsoft .NET Framework 2.0. Source is included if you desire to port it away from this prerequisite.

System WADs can be installed using any normal installer.

http://code.google.com/p/nisdownloader/