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.
