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.
Software Product Management Stack - Resources & tools to help you manage your software product
Planyway - Visual project management for Jira and Trello: manage resources, build project roadmaps, use time tracking.
The Product Management guide - A book on how to create and ship products that people love
Corrello - Dashboards for agile teams using Trello
Product Management Exercises - A repository of product manager job interview exercises