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.
HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
Toby for Teams - Better bookmarks, now with collaboration baked in 🔖
Rivers IM - Simple, fast & secure team collaboration platform
Planyway - Visual project management for Jira and Trello: manage resources, build project roadmaps, use time tracking.
Slack - A messaging app for teams who see through the Earth!