CoverMe - Private texting, calling, vault
Snarl - Snarl is a notification system for Windows.
Toasty - Photo messages that can't be missed
Growl for Windows - Growl for Windows is a Windows-compatible version of https://alternativeto.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.