This isn't a board game, but it's close, and I didn't want to make a new "video game" category just for Civ6.
Cowpoker: Record Sheet (PDF)
A handy way to account for every bonus, especially in a multi-round game.
Makes it easier to remember the goals. Print a second copy for the UFOs.
Contains a house rule raising Discordia's goal to six Weird groups, as later expansions made Weird the most common alignment by far. This is a good change. Use it.
A paper version, for the older edition.
A version that easily fits in the box.
A two-page reference for the cooperative Reiner Knizia game, including the Black Gate rule, an official erratum which fixes how easy it was to acheive a Military Victory.
Translations by BIGsheep and T. Dorst; I've just formatted it as a printable version.
A random season card generator, plus new official game modes from the comics
Questions and answers, along with a few ways to tweak the game.
Minor rules to fix specific issues
Resources, useful links, and house rules.