Disclaimer: I'm a Professional Scrum Trainer with Scrum.org.
There are two well-recognized certification bodies for Scrum in the industry, Scrum Alliance and Scrum.org. These have been around for a long time, their certificates are backed by a strong community, brand and they both descend directly from Ken Schwaber and/or Jeff Sutherland, who are the authors of the Framework and the Scrum Guide.
Scrum.org offers you the ability to take its Scrum Master assessments without having to participate in a training. They have two:
The CSM assessment from Scrum Alliance can, as far as I can tell, only be taken after attending a course.
In the last year or so a number of additional "certification organisations" have sprung up. ScrumStudy is one of them, but their "Scrum Body Of Knowledge" book has been heavily debated in the broader community, as it approaches scrum far more as a Project Management Framework with all kinds of processes and practices, than an empirical Framework.
With Agile being hot and a lot of money to be made in online-training and certification, a number of parties have stepped into this market.
I've personally read through the materials, even attended some of the courses online and my personal conclusion is that if it's real Empirical scrum you want, then these organisations don't really hit the mark. To me these "other" certificates are not of sufficient value that I'd associate myself with them.
My personal opinion is that if I've never heard of a certification body, or if their material, commercial practices (like very similar naming conventions) or anything else looks suspicions, I'd have trouble "trusting" any certificate I'd receive from them :).