Nifty - Manage projects, work, and communications in one place.
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.
Intercom on Product Management - The ultimate resource for aspiring product managers
UpDiagram - Free agile project management tool for scrum team
Jira - The #1 software development tool used by agile teams. Jira Software is built for every member of your software team to plan, track, and release great software.
productboard - Beautiful and powerful product management.