Nifty - Manage projects, work, and communications in one place.
The Product Manifesto - Product Managers face the biggest challenges of our day, which is why our community needs a common set of product principles.
Intercom on Product Management - The ultimate resource for aspiring product managers
Product School - The global leader in product management training
GitHub Project Management - No-fluff guide to building a collaborative development team
Product Management Exercises - A repository of product manager job interview exercises