The gateway application used to launch custom software on the Wii.
This is a newer alternative that can be a good option if you have a USB hard drive but not an SD card. It runs as an application from the Homebrew Channel and allows you to dump the NAND directly to a USB storage device [0†L6-L8]. The process is generally similar: run the app, select your destination, and wait for the backup to complete. Wii Nand Backup Files Download