Monitornator - Server monitoring made simple
Snarl - Snarl is a notification system for Windows.
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
Notifu - Notifu, a free open source pop-up balloon utility
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Notify OSD - Canonicals on-screen-display notification agent, implementing the freedesktop.