BootMii is a system designed by Team Twiizers to enable complete control of the Wii. It allows the Wii to be controlled about one second after the On button has been pressed, even before the System Menu. In fact, it can be controlled even before the IOS even loads.
BootMii can allow anything from Recovery modes (creating a practically unbrickable Wii), to lazy access of the Homebrew Channel. For example, if you have corrupted the System Menu, you can use the AnyRegion Changer to install a System Menu 3.2. Unfortunately, all of these tools need an IOS, because libogc needs IOS. However, there will probably be a Recover App made in the future at some point.
How it works
The BootMii Platform is a boot2 hack, which is loaded by boot1, which is loaded by boot0. boot0 is part of Hollywood and read-only. boot1 is signed by a value in write-once memory and therefore cannot be changed. However, boot2 is the first code loaded from the NAND. This means it can be hacked, and also updated, and corrupted. BootMii hacks the boot2 files, and allows us to run code straight from the SD Card, before anything else is loaded. This has huge advantages, such as making it very difficult to brick, and keeping Nintendo from stopping homebrew. The only way we could stop Nintendo from blocking homebrew completely however, is by using BootMii to patch the updates on-the-fly. Also, it may be possible to keep anything from overwriting the boot2 hack.
As you may know, BootMii has not yet been released. It may even take a few months. There is not a set release date, so DO NOT BUG TEAM TWIIZERS (e.g. MARCAN, BUSHING, ETC.) ABOUT WHEN IT'LL BE RELEASED! BootMii will be released when it's done. If made sloppily, it can and will brick your console. It is undergoing a lot of work, and it is being thoroughly tested. Remember how long it took to make the Homebrew Channel? Well, they are going to do even more testing, because of the nature of such a hack.
BootMii will not require any special hardware. However, special hardware might help accomplish things that BootMii by itself cannot, such as hardware NAND write protection and isolation from the Nintendo software stack. No such hardware exists yet though.
The new boot1
Nintendo has released a new version of Boot1, which disables Bootmii as a Boot2 hack. Fortunately, Bootmii can also be modified to work as a replacement for IOS, or as a separate IOS (Hypothetically, IOS5). See more at Hackmii. Since boot1 cannot be updated, all consoles already manufactured before this update are safe.
This is a demo from Marcan about the BootMii platform. This is (a very old version of) the official BootMii. Please read the video description if you are wondering about the hardware on his Wii.