Free modern poker training site - Zenith Poker
I stumbled across this the other day when looking for tutorials on the GTO+ solver. It's pretty amazing, I thought it was light on content at first but it's actually just free of cruft. It's a pretty clear explanation of the game theory math behind poker and how to apply it, does a much better job than my poker theory article. And seems to be totally free, I haven't seen any bonus content behind paywalls or anything pushing you to a product.