TinyNPS - Simple NPS feedback
Product Resources List - The biggest list of free product management resources
Interactive NPS Calculator (by Delighted) - Calculate your NPS easily with this interactive free tool.
Product Wednesday - Every Wednesday, receive a handpicked selection of tools made for Product Managers, Product Designers, UX Researchers and every member of the Product team.
Jaco - Watch and analyze your users’ behavior
Intercom on Product Management - The ultimate resource for aspiring product managers