There are two well-known Scrum Master Certifications in the Global Market
1. Certified Scrum Master (CSM) by Scrum Alliance
Prerequisites: Attendance of a CMS course by a certified Scrum Alliance partner
Cost: Included in seminar cost, which is rather pricey (last course in Greece in 2014, cost 950 Euros)
Certification test type: Online
Passing score: 24/35 Questions (69%)
Duration: No time limit
Expiration: Two years from test
Renewal prerequisites: None
Renewal cost: 100$, for 2 years.
Market perception: CSM was the first Scrum Master Certification on the market and had the 'first mover' advantage. Until March 2012, the certification was awarded to all course attendees regardless of score. The certification is still considered 'very easy' to obtain and, I believe, is perceived by the job market more as a 'certificate of attendance' of a Scrum Alliance course, and less as a professional certification. Nevertheless, it is by far the most popular. At the time this article was written, Dec 2015, 319.385 CSM certifications had been awarded.
2. Professional Scrum Master I (PSM I) by Scrum.org
Prerequisites: None
Cost: 150 $
Certification test type: Online
Passing score: 68/80 Questions (85%)
Duration: 1 hour
Expiration: Never
Renewal prerequisites: N/A
Renewal cost: N/A
Market perception: PSM I was introduced in 2011, much later than CSM. It is perceived to be more 'user friendly', (no class prerequisites, no renewal costs) but also more difficult to obtain due to the higher passing score. The job market, I believe, perceives it to be more 'valid' as a basic Scrum knowledge safeguard, in the sense that not everyone that takes the test passes. At the time this article was written, Dec 2015, 45.697 PSM I certifications had been awarded.
Other certifications:
Project Management Insitute's ACP (Agile Certified Practicioner) mostly follows the hugely successfull PMP model (Working experience and training prerequisites, multi-hour examination, serious renewal prerequisites), but with little success (9.625 PMI-ACP certificate holders at Sep-2015). The reason, in my mind, is fairly simple: PMI is perceived (and rightly so) as a "traditional" (waterfall) Project Management advocate, and not an active actor in the global Agile scene.
Prince 2 Agile is fairly new (introduced in 2015), and all others (scrumstudy.org, scrum-institute.org, ...) are largely insignificant.
