Passed the PSM-I exam today first time with 96.3% using only freely available resources. This forum was incredibly helpful so I'll share what I learned over the last two weeks since electing to pursue certification.
By way of background, I have extensive experience in software and product development and have worked in organizations with an Agile practice, but aside from attending Sprint Reviews I have not yet participated on a Scrum Team. That said, I did instinctively gravitate towards the Scrum pillars and values. If you don't know these, don't take the test yet :)