Pushbullet - Pushbullet - Your devices working better together
Snarl - Snarl is a notification system for Windows.
Pushover - Real-time notifications on your Android, iPhone, iPad, and Desktop
Notifu - Notifu, a free open source pop-up balloon utility
QPush - QPush is a free service that lets you easily push text and links from PC to iPhone.
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.