Snarl - Snarl is a notification system for Windows.

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.

Notifu - Notifu, a free open source pop-up balloon utility

Textra - Textra is a seriously beautiful, feature rich SMS and MMS app.

Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.

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.