Notifu - Notifu, a free open source pop-up balloon utility
Twinme - Twinme – Private Messenger is a free, secure, instant, and high-definition messaging app that protects your privacy along with giving you full control over each of your relationships and contents.
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.
Textra - Textra is a seriously beautiful, feature rich SMS and MMS app.
Growl for Windows - Growl for Windows is a Windows-compatible version of https://alternativeto.
Material Messaging - Material Messaging is a beautiful communication app that lets you send and receive SMS and MMS messages using the intuitive interface while giving you complete control over every aspect of your messaging experience.