Using a hardware device to trigger a built-in recovery mode in the Wii System Menu. This can be used to recover many otherwise bricked Wiis.
Nintendo calls the official version a "Waikiki adapter", and it seems to be a device similar to the USBGecko (An EXI to serial/USB bridge). Only its presence is necessary to trigger the recovery menu - you don't actually have to plug it into anything on the other end. The clone that I built only pretends to be a Waikiki, but so far does not offer any serial functions.