13 valuable online resources/apps everyone deserves to know about:

1. 7cups.com: free online therapy + you can help others too. 
2. Volunteermatch.org: find places to volunteer near you no matter where you travel.
3. Thriftbooks.com: cheap books.
4. Readanybook.com: more free/cheap books.
5. Freerice.com: give free rice to those in need while simultaneously increasing your vocab game.
6. 10minutemail.com: temporary e-mail address to sign up for sites.
7. Jstor.org: digital library of journals and primary sources (and I think they have books now). 
8. Thoughtcatalog.com: some short and fun articles to read if you're sitting in a waiting room.
9. Scale of the Universe: if you like animation and space/astronomy (There are two versions: Scale of the Universe 1 and Scale of the Universe 2 - not sure which this one is.)
10. Fastweb.com: if you're on a scholarship hunt. (It's free but you have to create an account for this.) 
11. Canva.com: if you need to make a quick and easy advertisement or creative holiday card.   
12. http://sciencecases.lib.buffalo.edu/cs/collection/results.asp?subject_headings=Psychology: psych case studies, if you're interested in that kind of stuff. Each case comes with an answer key, but allows you to answer/think through it first.
13. https://www.nlg.org/resource/know-your-rights: short and helpful guide to knowing your basic rights, US citizen or not (comes in different languages). This is not a substitute for legal advice.

There are obvious limitations to all of these sources, especially the last, so please feel free to add to the list by commenting below.

P.S Wanted to add a guide about saving and investing for teens and people my age, but as of yet, I don't know any credible sites. I do have one book, though. Check it out: The Motley Fool Investment Guide for Teens by David and Tom Gardner. Worth a read, it's short and light!