Slack
Slack is a messaging suite aimed at businesses and offices. Meant as an alternative to emails, messages can be sent to groups, individuals, or posted in threads. It offers file sharing and voice and video calls as well as integration with other pieces of software such as google drive. Slack is a great way to keep information organized by department and keep quick questions out of email inboxes.
COST:
Free tier missing some features
Pro at $6.67 / user / month
Business+ at $12.50 / user / month