Getting IntelliJ IDEA for free is best done through rather than looking for serial keys, which are often unreliable or bundled with security risks. Here are the legitimate ways to access IntelliJ IDEA:

If you lead an active open-source project, you can apply for a free license for your entire team [4].

You can download the full Ultimate edition and use it without a key for 30 days to test out all features [2].