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.
A list and description of every spirit in the game (as of 2022) to help players choose theirs.
Both the board game and video game.
Minor rules to fix specific issues.
Resources, useful links, and house rules.