Persyst

Persyst is a runtime serialization plugin for Unity.

It's not quite a deployment-ready save system, but rather a toolset with which to build your own, as simple or complex as you want it to be.

That being said, very little code is required to make a simple save system out of it — and it's mostly just tagging fields with attributes, like this:

using Persyst;

public class SerializableMB : MonoBehaviour, ISaveable
{
    [SaveThis] int value;
}

For a far more detailed explanation of what this is all about, see the docs.