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.
Notix.co - NOTIX is an audience re-engagement service based on web and in-app push notifications that work for both desktop and mobile devices.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Airship - Mobile marketing solutions across all major mobile platforms.
Slack - A messaging app for teams who see through the Earth!
OneSignal - Customer engagement platform used by over 1 million developers and marketers; the fastest and most reliable way to send mobile and web push notifications, in-app messages, emails, and SMS.