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.
Instagantt - Instagantt is a powerful and intuitive Gantt chart tool to enable teams to plan, manage and visualize their projects easily. Manage your schedules, tasks, timelines, and workload like a Pro. Try it for free.
Rivers IM - Simple, fast & secure team collaboration platform
Kanbanier - Clean iOS and Mac kanban/trello with flexible views
Slack - A messaging app for teams who see through the Earth!