HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
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.
Rivers IM - Simple, fast & secure team collaboration platform
KanbanFlow - KanbanFlow is a Lean project management tool allowing real-time collaboration between team members. Supports the Pomodoro technique for time tracking.
Slack - A messaging app for teams who see through the Earth!
Meistertask - MeisterTask is an intuitive task management and collaboration tool.