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Product Manager Handbook - Interviews w/ product managers + weekly newsletter
Miraheze - Miraheze is a wiki farm (hosts wikis) for free and with no ads, it also provides custom domains...
PM Starter Pack - Practical guide on how to get started in product management
Investopedia - Investopedia has a lot of information to share with business mind people.
Everything.PM - 300+ resources & tools for PMs, sorted by votes