Hadoop is an Apache project, which means it's open source in nature. It's also a popular framework based on Java that's designed to support distributed storage, especially in connection with distributed processing of humongous data sets on generic computing clusters. As such, it's also an important element in many Big Data projects and activities. Given how popular Big Data is nowadays, no big stretch is needed to see Hadoop in the same light.
Hadoop's roots trace back to Dr. Seuss "Horton Hears a Who" to help explain the pachyderm
I decided to look around to see what kinds of Hadoop certification were currently available, and came away impressed with the number of options available, many from well-known names in the training game and/or those behind or involved in creating and maintaining Hadoop itself (especially for commercial use). Here's a smattering of players and credentials, with links to match:
A training and technology company with deep roots in the cloud and Big Data, Cloudera offers a general Data Scientist credential, plus Hadoop Developer and Hadoop Admin certs as well. Udemy offers a training tutorial for the Hadoop Developer (CCDH) cert, too.
A key driver in the Hadoop community, Hortonworks offers a slate of three certifications for Hadoop as well: HDP Certified Developer, Hortonworks Certified Apache Hadoop Administrator, and Hortonworks Certified Apache Hadoop Java Developer.
MapR Academy, a unit of MarpR Technologies provides a unified distribution that combines Hadoop, NoSQL, database and streaming applications used by hundreds of global customers, whose partners include Cisco, Amazon, Google, and Teradata. Their certifications also include Administrator and Developer elements, with a Data Analyst element on its way sometime in the next year.
Lots of other players offer classes and/or tutorials, some of which (like those from Big Data University) even come with certificates of completion (not the same as a certification, but easy to confuse with same). There is no shortage of opportunities to learn more about Hadoop, though it's probably best to stick to the big-name credentials if you want certs with the most amount of oomph to repay the time, money and effort involved in earning them.