Bannerbomb is currently the most popular exploit for hacking your Wii. This is mainly because its the only exploit that can be launched from the wii menu itself, without you having to buy a certain game.
It uses a malformed banner to cause a buffer overflow (as it fails to load the malformed banner, similar to a banner brick). Like this, it can load any boot.dol or boot.elf file that can be found in the root of the SD card.
If a boot.elf and a boot.dol are in the SD card root at the same time, Bannerbomb will fail to load.

The system menu 4.2 update blocked the old version of Bannerbomb. A new version was made that uses the same exploit, but on another place (the SD card menu that was added in update 4.0).
Since the new bannerbomb will only work with the SD card menu, it can only be used on System menu 4.0 and higher.

Required:
* An SD card (Best results are with non-SDHC cards, SDHC will only work on 4.0 or above) formatted to FAT(32).

Links:
* Bannerbomb v1 (system menu below 4.2): http://bannerbomb.qoid.us/index.old.php
* Bannerbomb v2 (system menu 4.0 - 4.2): http://bannerbomb.qoid.us/index.new.php

Note: The author of bannerbomb does not like it when other people mirror his links. Please link to the bannerbomb site instead.

Guide:


Computer ----------------------

1. Go to the bannerbomb website (use the second link if you have System menu 4.2 and the other link for all the older versions). You will see some steps there, which you are free to follow, as this guide basically tells you the same.

2. First of all, we need to rename our private folder (this is because bannerbomb can only work if its the only file in the private folder). You can for example rename it to "privateold" or something similar. Of course you can also move the private folder to another place.

3. Download the first version of bannerbomb (aad1f_v108.zip for 4.1 and lower, abd6a_v200.zip for 4.2). Extract the zip file to the root of the SD card. You should have a private folder again, with only bannerbomb in it.

4. Download any wii app and place its .dol or .elf in the root of the SD card. Be sure to rename it to boot.dol/.elf


Wii ----------------------

5. If you enabled WiiConnect24, go to your Wii settings and disable it. This increases the chance for success (due a bug in bannerbomb)

6. Now we need to execute the bannerbomb exploit.

• On 4.1 and lower: from the Wii dashboard (the screen with the channels you see after the health screen), press the big WII button at the bottom left. Now go to Data management, Choose Wii, and then go to the SD card. Since bannerbomb is on our SD card, it will execute automatically

• On 4.2: go to the SD card menu and bannerbomb will execute.

7. After step 6, there are 2 things that can happen.

• If it freezes, go back to the bannerbomb site and download the next version. You can find the next versions under step 6 of the bannerbomb site.

• If it shows "Load boot.dol/.elf", it works and you can click YES to run it.
  

Note: if the wii freezes after the "Load boot.dol/.elf" prompt, this is not caused by bannerbomb but the app itself !