productboard - Beautiful and powerful product management.
Hassl - A really great project management app
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.
Arbor - Easily manage product development
Roadmap - Collision avoidance for projects and people
Slack - A messaging app for teams who see through the Earth!