Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Product School - The global leader in product management training
Planyway - Visual project management for Jira and Trello: manage resources, build project roadmaps, use time tracking.
The Product Manifesto - Product Managers face the biggest challenges of our day, which is why our community needs a common set of product principles.
Corrello - Dashboards for agile teams using Trello
Intercom on Product Management - The ultimate resource for aspiring product managers