Pushover - Real-time notifications on your Android, iPhone, iPad, and Desktop
Snarl - Snarl is a notification system for Windows.
Pushbullet - Pushbullet - Your devices working better together
Notifu - Notifu, a free open source pop-up balloon utility
Gotify - a simple self-hosted server for sending and receiving messages
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.