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

Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.

Party Line - Use Slack to manage your projects

Basecamp - A simple and elegant project management system.