Wii U

IMPORTANT:
Set the DNS to 168.235.092.108 and 081.004.127.020 to block the console from updating itself.
If you don't do that, you'll allow Nintendo to update your console without asking.

GUIDES
[Start here] Haxchi/Mocha guide: https://wiiu.guide/
NOTE: Installing coldboot haxchi may brick your console (if you don't know what you're doing), but is also optional, so you can skip that step if you're unsure.
Old guide: https://github.com/FlimFlam69/WiiUTutorial/wiki

PIRACY
Brazilian Method (recommended, installs legit games to your HDD):
https://github.com/FlimFlam69/WiiUTutorial/blob/master/btimsg.md

Loadiine (not recommended, long loading times, incompatible with some games, only useful if you want mods):
http://pastebin.com/HvHDL1mD (skip to "SD card setup")

USEFUL TOOLS
Dumping disc-based games for installation: https://wiiu.guide/disc2app
vWii modding: https://wiiu.guide/vwii-modding
NAND backup: https://wiiu.guide/nand-backup
Inject Wii/GameCube games via Virtual Console: https://gbatemp.net/threads/release-wiivc-injector-script-gc-wii-homebrew-support.483577/
Inject (S)NES/DS/N64/Wii games via VC: http://gbatemp.net/threads/release-ultimate-vc-injector-for-wiiu.486781/

DLC in Loadiine: http://www.wiiuiso.com/nintendo-wii-u-tutorials/389768-loadiine-gx2-mod-hypocrisy-free-edition-tutorial.html
Loadiine comptability list: https://wiki.gbatemp.net/wiki/Loadiine_compatibility_list
SSB4 mods: http://pastebin.com/ES053DpJ

FAQS
>How do I update to 5.5.1 now that 5.5.2 is out?
https://gbatemp.net/threads/updating-to-5-5-1-or-5-5-2-from-lower-versions.478867/

>How can I get VC games to show up in Loadiine?
Just stop using Loadiine.

>Can I use DLCs and play online?
Unless you're using Loadiine, yes. Keep in mind that you _will_ need to run CFW in order to play DLC and eShop-only games.

OTHER STUFF
Splatoon mods: http://gbatemp.net/threads/425670/
DS VC layout editing: http://gbatemp.net/threads/426862/
In DS Virtual console games you can change the internal resolution to X2 by editing the /game/content/configuration_cafe.json file.
Search for "Renderscale", it should display a 1. Switch it to 2.