Final Fantasy 14's second major expansion, Stormblood, is free to download right now for the next month.
As highlighted on the game's website, players can claim and download Stormblood for free, right through to May 8. There is one slight catch, as you need to be an existing owner of Final Fantasy 14 Starter Edition in order to do so. It's quite the big expansion to be giving away for free, and considering the Starter Edition also includes the game's first expansion Heavensward, you'll be able to get through a big chunk of the main story for not too much money.
The expansion will be free on all platforms too. On PlayStation you just need to find the Stormblood purchase page on the PlayStation Store, and it will be free as long as you own the Starter Edition. And for PC/ Mac players, you can just purchase it for free from the Square Enix Store or Steam. Thankfully Square Enix has set up a little page to show you all the instructions for redeeming the expansion just in case you're uncertain.