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
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Slack - A messaging app for teams who see through the Earth!
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.