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
AirDroid - Access Android phone/tablet from computer remotely and securely. Manage SMS, files, photos and videos, WhatsApp, Line, WeChat and more on computer.
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.