Here's something you don't see too often: a ransomware creator unearthing the master decryption key for public consumption. That's exactly what we're seeing from Petya's original developer, allowing those affected by certain versions of Petya to recover their data, and developers the ability to create decrypters to make the entire process that much easier.
Unfortunately, there are a number of major caveats here. The biggest one is the fact that most of those affected by these specific versions of Petya dealt with it last year, not recently. It stands to reason that many of those folks did not clone or keep their drive, because it could have felt like a lost cause. If you still have the data, you should now be able to decrypt.
Another major gotcha is the fact that this doesn't aide those dealing with the more recent NotPetya spin of Petya (which was originally called Petya, confusing matters). Petya's creator, a group calling itself Janus Cybercrime Solutions, has denied any involvement with NotPetya. It's actually believed that the release of NotPetya is what caused the Janus group to cough up the master key for older versions. Janus denies involvement in NotPetya; it's an unofficial fork that changes a few things up.
If you were affected with Petya which featured a white skull on a red background, green skull on black background, or yellow skull on black background, this master key release will help get your data back. If you haven't been bit by Petya, it's important to continue doing your due diligence in avoiding getting struck by this kind of headache. We should really be mimicking Steve Ballmer's "Developers, developers, developers!" chant to instead say, "Backups, backups, backups!"