Steam frequently hosts Free Weekend events for popular games. During these periods, you can download and play the full version of Planet Zoo for free for 48–72 hours.
Beyond the ethical argument of supporting developers (Frontier has poured years of free updates into Planet Zoo), there is a practical reason to buy the game legitimately or via a deep discount:
The Steam Workshop is half the game.
Planet Zoo’s community creates thousands of blueprints (habitats, scenery, entire zoos). To download these, you need a legitimate copy of the game tied to your Steam account. Pirated or "free key" versions cannot access the Workshop. You will be missing out on the game's best feature: the creativity of millions of players.
Since a truly free Steam key is a myth (outside of winning a lottery-odds giveaway), here is the smart player’s guide to playing Planet Zoo for the lowest possible cost.