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.
UI Movement - The best UI design inspiration, daily
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.
Design Hunt - Curated design inspiration for everyone
Basecamp - A simple and elegant project management system.
UI Jar - Hand-picked design inspiration for your real life projects