Everything.PM - 300+ resources & tools for PMs, sorted by votes
LetterHunt - LetterHunt connects you to 10,000+ niche, active newsletter writers to promote your product.
PM Starter Pack - Practical guide on how to get started in product management
Mattermark Daily - A curated newsletter of blog posts from investors & founders
Product Manager Handbook - Interviews w/ product managers + weekly newsletter
Dayblizz - Dayblizz is a social media app for creating, sharing and monetizing content