Intercom on Product Management - The ultimate resource for aspiring product managers
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
GoodGantt - The best Gantt chart for Trello®
The Product Management guide - A book on how to create and ship products that people love
Paws for Trello - The best way to use Trello on your desktop