Apple and Microsoft have their own courses, which give you a certification if you pass them. I could not find a definitive answer on whether Ubuntu Online courses, or any other Ubuntu training programs, exist.
Here on Ask Ubuntu there is a question about Ubuntu Certified Engineers, but that question and its answers haven’t been updated in years; they’re outdated. Even if they weren’t, they don't really answer my current question.
Is there a way to get myself certified as an Ubuntu Engineer or specialist, or for any other role?
As I understand the certification universe, there is no "Ubuntu-specific" certification. Mainly, the general universe of execs hiring IT Professionals don't look for just "Ubuntu Certification", more often they're looking for Linux expertise rather than just one specific OS. Sometimes they want expertise in the specific OS, and in fact in one workplace I know the sysadmin is CentOS/RHEL experienced, and one of the other staffers is Debian/Ubuntu experienced, and they both share linux administration, not to mention teach each other things from time to time.
If they're looking for a Linux certification, they're looking for a general certification. Here's some of them. This is NOT an all-inclusive list:
Linux Professional Institute Certifications and such exist too, separate from the Linux+. [general link]
Linux Essentials is their most basic 'certification', and isn't really a systems administrator certification - it merely covers the absolute basics. I wouldn't even call it a certification worth anyone's time. Details about this one are available on the LPI site.
LPIC-1: Linux Server Professional Certification. This is my current (as of July 2015) level of LPIC certification, and it is a much harder certification than Essentials. This covers a lot more than the bare minimum to understand Linux. The details of what it covers are listed in the two exams' focuses that cover the certification. LPIC-1 information is available here, and is more of a systems administrator certification. Taking the LPIC-1 also gets you an OpenSUSE administrator certification if you pass.
LPIC-2: Linux Network Professional Certification. This is the next step up from LPIC-1. It's a networking and advanced systems administration cource. Details on what LPIC-2 covers is here. You'll need LPIC-1 before you get LPIC-2.
LPIC-3: Linux Enterprise Professional Certification. This is by far the most difficult, but also the most specialized of the LPIC exams. Provided you have LPIC-1 and LPIC-2, you must pass one of three potential exams, which focus on special-focus exams. Their site goes more in depth on these, but here's a short summary:
Exam 300: Mixed Environment. (Corresponding certification designation: LPIC-3 Mixed Environment) This exam focuses on mixed-environment areas, and different tasks such as: OpenLDAP Configuration; OpenLDAP as an Authentication Backend; Samba Basics; Samba Share Configuration; Samba User and Group management; Samba Domain Integration; Samba Name Services; and, Working with Linux and Windows Clients.
Exam 303: Security. (Corresponding certification designation: LPIC-3 Security) Pretty self-explanatory, this exam focuses on security topics, such as: Cryptography; Access Control; Application Security; Operations Security; and, Network Security.
Exam 304: Virtualization and High Availability. (Corresponding certification designation: LPIC-3 Virtualization and High Availability) This focuses on tasks and items such as: Virtualization; High Availability Cluster Management; and, High Availability Cluster Storage.
While each of these certifications has their quirks and difficulties, these are some of the most popular certifications I have seen.
The Linux+ certification is a nice starting point. So is the LPIC-1. If all you're after is a certification to say you're competent as a Linux sysadmin, that's probably all you'll need to be considered a server administrator.
The more specialized certifications will indicate you are a more global Linux Professional, but usually that's not necessary if you're after just a plain old sysadmin position, provided you can demonstrate ample knowledge to back up your skills.
Other certifications not in my list
Linux System Administrator Certification, from LinuxFoundation.org (Thanks to @Wilf)
Linux Foundation Certified Engineer, from LinuxFoundation.org. (Thanks to @Ron)