Learn to play video:
Learn to play booklet:
Living Rules Reference (includes some errata/clarifications/etc):
Tool for randomizing race selection:
Tool for generating roughly-balanced maps:
Useful for randomizing player lists (for starting location, race selection, etc):
Drop a line in here if you have other useful sites and I'll update the top post once I've had a chance to check them out.
Here are a couple I want to look at when I have the time:
https://ti4-map-generator.derekpeterson.ca/Edit: looked at this, it looks spectacular, moved it up
I've been studying this spreadsheet all week long.
This is pretty sweet.