For sites, Livius.org is a great, often accurate site for Greco-Roman articles that isn't a copy-paste of Wikipedia or some generic short summary from some University course textbook.
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For free-texts, Perseus Project by Tufts University has most extant major Greco-Roman works up until late Antiquity: http://www.perseus.tufts.edu/hopper/
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For a great free textual introduction to Greece, Perseus has on their site *An Overview of Classical Greek History from Mycenae to Alexander* by Thomas R. Martin, which gives a summary of events and background of Greek civilization from the late Bronze age to Alexander the Great: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0009%3Achapter%3D1
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There's three great and relatively short University courses on Coursera relating to Ancient Greece, one is a General history, another focused on Greco-Roman myth, and one an introduction about major Ancient Greek philosophy schools and philosophers (Pre-Socratics, Plato, Aristotle, Cynics, Epicureans)
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General History course: The Ancient Greeks (Wesylan University; around 23 lecture hours long): https://www.coursera.org/learn/ancient-greeks
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Greco-Roman Myth: Greek and Roman Mythology (University of Pennsylvania; around 31 lecture hours long): https://www.coursera.org/learn/mythology
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Introduction to Greek philosophy: Plato & His Predecessors: https://www.coursera.org/learn/plato Aristotle and his Successors: https://www.coursera.org/learn/aristotle
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There are no good trustworthy Youtubers that have covered Ancient Greek history in my experience. I'm just putting that out there. The best there is are those who know their own limited scope and just stick to doing rather generic military history battle summaries who attempt to limit their own bad-history. Youtube just isn't a medium credible people who know topics relating to Ancient Greece use. Stick to books and stuff written by credible academics online.
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For a good lecture series on Alexander the Great: Robin Lane Fox - " Alexander the Great (14 parts): https://www.youtube.com/watch?v=JGpIrd-Ek5k&list=PLhabqLEle0TUPyL1GC0pHS5p8u89nY0bU
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For topics and subjects relating to the Greeks contemporary Near-Eastern neighbors, Encyclopædia Iranica is a great site (especially for history of Iran and Iranic people): http://www.iranicaonline.org/
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/u/Iphikrates also has written some very excellent answers to innumerable amount of questions relating to Ancient Greece that might fill in some common gaps from the above (especially for topics concerning for Classical Greek warfare): https://www.reddit.com/r/AskHistorians/wiki/profiles/iphikrates
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