• MasterClass
  MasterClass is a bit different because it does provide access to "classes" with celebrities rather than academic field experts. You have a bunch of teasers you can view for free (and you should watch them, to get an idea what you will spend your money on), but then you need to invest - they don't offer free courses. What you get is an insight into the thoughts and advice from people that are well known in the field of their profession. You can take a "master class" with Carlos Santana about the art of soul guitar, a cooking class with Gordon Ramsay, listen to the advice of Niel deGrasse Tyson about scientific thinking or learn about how did Hans Zimmer works on film music as the one for "Interstellar" or "Sherlock Holmes".\ MasterClass offers more than 80 "classes" from high profile celebrities with fascinating insights. The video quality and the setting in which the classes have been filmed is exceptional.\ If you are interested not only in acquiring skills but also in the thoughts of some of the people giving the classes on this platform, then the investment of about 200.-- € is definitely worth it (that's the price for a 1 year subscription). You will get first-hand information from the "teachers" that you will get nowhere else. If you expect to get tests and certificates, this site is not for you.
• coursera
  Offers free courses from currently (2018-10-16) more than 150 universities all over the world. The tests differ a lot in complexity. Most of the courses offer free content, but paid learning-checks and certificates - in the past, they did offer a free "Statement of Accomplishment" (which seems to have been discontinued) and free learning checks (this has been changed 2016 to only be part of the paid option). Some courses now require a monthly subscription - the slower you are, the more expensive the course becomes.\ Most courses require completion of some multiple choice tests to get a certificate. Some courses also use peer-reviewed text submissions for grading. Many courses support download of videos.
• udemy
  Paid courses from individuals with prices of 0$-299$ US. The quality of video and course material is highly heterogeneous. You should take the reviews into account before buying a course. In some courses, you'll get the certificate by just browsing through the videos, for other certificates you need to perform some tests. They are offering discount codes (up to 90% off) sometimes for a selection of courses, sometimes for all courses. Download of videos is not supported, but manageable with a video downloader tool.
• Cybrary
  Mix of free and paid courses of different level, quality and length. They offer some content from Cisco and other well known companies. Interesting about this one is the offer of articles, events and forums - they aim to build a rich community in the field of cyber security. Also, they offer a lot of job opportunities and want to help you getting a job in the cyber security industry.
• PluralSight
  Paid IT course provider. PluralSight is not a MOOC provider since they are offering a course library (really many courses) of already recorded videos with some simple multiple choice tests. The tests I took were straightforward and definitely do not certify a deep understanding of the field, but only that I did view the videos. Download of the videos is explicitly forbidden - it's possible, but because of the short video sequences with cryptic names not practical. They offer a separate extra premium for offline viewing with a proprietary player and access to additional course material (source code, documents, etc.). You can have a limited test account (no offline viewing, max. 200 minutes video, max. 3 certificates) for free, but you need to specify a credit card even for the test account - if you forgot to cancel the test account, it would be converted into a paid one. There is an option for a company account, which offers the account administrator access to learning statistics about hours invested by individuals, success rate, etc.. Because of this, you should play the whole video if you want to demonstrate the value of the subscription to your boss.
• edX
  Partners Harward and MIT to provide free courses from different universities. Currently (2018-10-16), they are offering about 2400 courses about all kinds of topics. It's a non-profit created by founding partners Harvard and MIT. The courses feel much more interactive because of a more detailed structure in the UI. The content is mostly high quality. They also discontinued the "honor code certificates" (free) for new courses and asked for (optional) donations when enrolling without paying. The free versions of the courses are called "Audit this course" - you can later convert it to a paid enrollment. Verified certificates are available from about 50 $US upwards; some courses are offered as "professional education" (like "Legal Risk Management Strategies for Multinational Enterprises") for much higher prices (I already saw prices up to 950 $US).
• LinkedIn Learning
  LinkedIn did acquire lynda.com in 2015. In 2016 LinkedIn has been acquired by Microsoft and later 2016 LinkedIn started offering video courses. Until now LinkedIn Learning does not offer a PDF- or other printable certificates of completion for a course, but you can add the course to your LinkedIn profile with only some clicks. In some videos, you still can see the Lynda-branding of the courses. There are some courses with quizzes. LinkedIn Learning is a paid "flat-rate" feature of LinkedIn, so you need the premium subscription of LinkedIn to use it - but currently, they are offering 1 month for free (for testing purpose).
• FutureLearn
  FutureLearn does provide the courses for free and offers paid certificates (2 types of certificates). The courses do contain quizzes, and you should be aware that all quizzes do contribute to the final score (something that I learned in my first course at FutureLearn, when I was still experimenting with the test process). The courses seem to be of good quality. The certificates are delivered printed on paper by traditional mail - the "digital" version is just a link to the website with an excellent rendered website that shows your test scores ... this might not be the way you want to present the certificate to your employer, if you just reached the minimum amount of points to get the certificate.
• Udacity
  About 30 currently free courses (most IT and Math). I'm currently taking my first course. They seem to do a little more intensive interaction with questions while you are viewing the videos. They aim to be more interactive than others and want to provide "affordable courses" - what means in the end: paid in the future. Currently (2016-06-12), you can enroll to "nanodegrees" where you pay per month (e.g., 200,- € per month for "Senior Web Developer" with an estimated average time of 378h to completion - whatever that means).
• openHPI
  Free but German language only. The site is managed by the German Hasso Plattner Institute in Potsdam. Currently, they offer only a single course - 4 have been offered in the past.
• lecturio
  Most courses paid, some free but most in the German language. The website has a good user experience. The price for a course ranges from € 399,- to free (many courses offer a monthly fee of about € 5,-). Some courses can be downloaded via iTunes (Mac & PC, the videos will become inaccessible when the license for the course is not valid anymore).
• OpenCourseWorld
  Videos are hosted on YouTube and cannot be viewed offline / platform buggy and terrible user experience - canceled evaluation after 1h