HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
Quip - Quip is the modern productivity suite that simplifies your life and helps your team get work done faster.
Rivers IM - Simple, fast & secure team collaboration platform
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Slack - A messaging app for teams who see through the Earth!
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.