- Warning: Don't update from 5.5.1 if you want to do this! - Important Note: This is the current latest firmware, so you are safe to update right now, but avoid any future updates unless you are told it's safe.
Update Dec 12th:
So Wii U homebrew has had a massive boost lately, and it's probably also the easiest setup from a user perspective that there has ever been.
That is literally all you need.
Another option is this Android app, which lets you host your own server.
How to set it up!
First you want to download the Homebrew Launcher from here.
This goes on the root of your SD card, so you should end up with /wiiu/apps/homebrew_launcher and then the three files it uses within that.
Once you have this on your SD card and inserted into your Wii U, you browse to the aforementioned http://loadiine.ovh/ and hit go. That's it, you're done! Welcome to the HBC on Wii U.
Set any two of the following to the 'Primary DNS' and 'Secondary DNS'.
168.235.092.108 (ns3.titledb.com, hosted in LA)
081.004.127.020 (ns2.titledb.com, hosted in Netherlands)
What else can I do?
Well first off, you can use DDD to dump your own games. This includes discs, eShop titles, and Virtual Console games.
Then you can use Loadiine GX2 to launch your backups from your SD card.
A tool called HID to Vpad will allow you to use any USB compliant controller to play Wii U games. Want to play NSMBU with a Dualshock 4? No problem!
Anything else cool?
Eventually we should see emulators such as Retroarch running in Wii U mode, as well as all the neat little homebrew things that always crop up.
One thing you can do right now is inject your own ROMs into Virtual Console titles. This is especially easy for DS titles which are simply a zip file containing the ROM you want to run. Getting Final Fantasy IV up and running was a doddle, and you can even force a higher internal resolution which is not an option Nintendo normally give us access to. Check out the difference in these shots!
So how do I do that DS thing?
Once you haved dumped one of your existing DS VC titles using DDD, you will find it contains a file called rom.zip in the folder \vol\content\xxxx. You can simple replace the file inside this with a DS ROM of your choice and it will function, but you must ensure you keep the name the same. WUP-N-DAAP.nds was the one in the Brain Training I dumped, so I renamed FFIV to that and copied that in there in its place.
Also in this folder is a file called configuration_cafe.json. In there you will have, right near the end, "RenderScale": 1 and changing that 1 to a 2 is all you need to do to enable 2x internal rendering.
When you are done, create a folder in the /wiiu/games directory on your SD card with the format "Name of Game [xxxx01]". Make up anythingfor the four xes, but 01 must be present. So my FFIV is "/wiiu/games/Final Fantasy IV [DMSK01]"
Click the banner for a more detailed guide which now includes GBA too.
Update 25th May: A new tool. GBA2VC, has been released. This will make injecting GBA a lot easier.
[This post will be updated as new things come along or people like Rich correct anything here]
Update 15th August:
This will print your console's OTP region to the screen. Do not share it. The readout contains console-specific encryption data, which is property of Nintendo. It has NAND keys, the Ancast key, the common key, and others.
Download the file, place it in the a wii u/apps/iosu folder on your Wii U SD card and boot up the .elf from the Homebrew Loader. I can confirm that it works (after a few tries). Also...
DON'T POST YOUR OTP ONLINE. It's copyrighted material.