Bannerbomb is an exploit capable of launching homebrew from any version of the System Menu. It was created by Comex to replace the Twilight Hack, which does not work on System Menu 4.0.
The original Bannerbomb exploit does not work with System Menu 4.2, it only works with System Menu 4.1 or older. However Bannerbomb v2 has been released by Comex, and works on 4.2
The exploit works by using a malformed banner to crash the Wii and run code to load the boot.dol placed in the root of the SD card. It can be used to install the Homebrew Channel, DVDx and the BootMii public beta via the HackMii Installer.