LetterBomb is a safe way to enable homebrew on a Wii without hardware modification. LetterBomb is achieved by use of the Wii Messageboard which executes a homebrew application from an external SD card. Examples of such homebrew .elf or .dol files can be found on the Homebrew applications page. LetterBomb was created by Team Twiizers. This exploit only works with any region of System Menu 4.3. This exploit works without any game in a manner very similar to the now defunct Bannerbomb.

More information here: http://hackmii.com/2011/08/letterbomb/