What online educational services for students do you know/use?
There are many such services. What exactly help do you need? As for me sometimes I use writing services when I don't have enough time to do my assignment. For example recently I used Paperell service(this link) to write my literature review. I always was bad in such writing assignments like essays and other...
Uh, that's a tough one (I mean, selecting only few):
I second Khan Academy recommendation. If you have more time, investigate Coursera. See also MIT Free Online Course Materials, MIT Video, OpenCulture ...
Obviously, Wikipedia is excellent source of quick info, enought to get you oriented. Then follow the links to the sources the article cites. If you are reading on a controversial topic (e.g. anything to do with politics), be sure to check the talk page for dissenting opinion. No such "second opinion" is available for The World Factbook, but it is a valuable resource, nevertheless.
Encyclopedia of Life is an excellent source of expert-written articles on, well, life. For math lovers there is plus.maths.org; FQXi Community explores a gray area between physics and philosophy in a very accessible way.
There are many science news sites, like ScienceNews, Science Daily, IOP News... Most popular science magazines have free web content; I like Scientific American Blog Network and Blogs - New Scientist, but "mainstream" news outlets are also quite good: BBC, The New York Times Science ...
Science - YouTube is choke-full of videos on science and other interesting topics, both amateur works and professional educational or documentary content.
Many scientists and layman enthusiasts maintain interesting sites and blogs, from focused single topic to very eclectic ones. Google and Bing are both your friends. :o)
Now, if you are interested in politics and/or what happens in the world, the choice is overwhelming, both in the sheer volume and the range of quality and, um, orientation (i.e. objectivity and the particular choice of favorite prejudices). My personal favorite is The Guardian.
I could go on all day. If you add particular topics you are interested in to your question, I will add some more sources.
There are lots of them actually. But if I have troubles with my writing tasks I use services of professional writers. For instance, here https://edusson.com/do-my-assignment writers can do your assignment. That's quite easy. You write to them, discuss, and wait. By the way, thanks for the recommendations. I'll take a note of some of those you mentioned.
Students should use online tutorials and courses provided by some sites like Udemy, Udacity, Alison and Lynda.com.