Most of them have to do with obtaining educational resources.
Places where you can find books/information, a lot of it for free:
Internet Archive.
Taylor & Francis
Free eBooks-Project Gutenberg.
Library Genesis.
Jstor.
Research that has to do specifically with science:
PubMed.
ResearchGate.
The Lancet.
ScienceDirect.
Bioknowledgy.
Bioninja (often used by IB children).
WolframAlpha.
Thesaurus and word tools:
WordHippo.
Reverso and Reverso Context.
Read papers faster:
Unriddle.
"Free courses". You can take a lot of them for free but if you want to get certified often times you need to pay:
Cousera.
Write letters to your future self:
FutureMe.
