Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Growl for Windows - Growl for Windows is a Windows-compatible version of https://alternativeto.
Ripcord - A desktop chat client for Discord and Slack
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.
Done Hui - No need to switch between multiple pieces of software to get through the workday. CHATS: Communicate freely. CALENDAR: Know your team's availability, plan meetings. No more conflicts. TO-DOs: Stay on top of all projects. FILES: All files, one spot.
Notifu - Notifu, a free open source pop-up balloon utility