The Wii U Kernel exploit for 5.5 that has been left right and center of discussions on the Wii U scene recently, was just leaked a couple of hours ago on GBATemp.
This exploit lets you run Loadiine (Wii U backup launcher), Cafiine, and a bunch of other cool tools.
The exploit is compatible with (at least) firmwares 5.5.0 and 5.5.1.
Developer NWPlayer1234, one of the people behind the exploit (along with Marionumber1) has confirmed the leak is the real thing. It's a full fledged exploit and people have already been confirming they can run Loadiine and other tools with it.
Now this is more like a 'userland' jump to 'execute code' stage loader, so its messy and not easy thing to setup, so I suggest you visit Wololo's Blog which is linked below for more info, some 'how-to' videos if you wish to try this release out and don't get upset if not all games work and that you need to use your PC with Java installed, as this is basically a pre-release-alpha-testing package designed for other developers to try out and see how it works, not a finished-polished-off-package for the average joe Nintendo Wii U owner.