notification-thing is a full-featured implementation of the Desktop Notifications Specification. It is not tied to any specific desktop environment, and is focused on features like rate-limiting and flexible filtering to minimize distraction and noise ratio in a transparent way, without silently dropping anything ever.
MOTDstat (Message Of The Day - System Status) will dynamically generate the /etc/motd file with current information about system status and usage. Using crontab, the script will periodically display the status of system resources and services. The original message of the day will be stored in the /etc/motd.orig file. Configuring an email address, you'll receive notifications about the following issues with configurable warning and critical limits: high CPU load, exceeding disk space, intensive usage of swap space, a specific process or network service not running, NTP not synchronized, and too many messages in the mail queue.
WebMon is a Java VM monitoring tool. It can be deployed to any servlet container or embedded in your EAR, or in JavaSE applications. RSS, e-mail, and Jabber notifications of potential problems are sent. It is useful if you aren't able to use traditional tools such as JConsole or VisualVM.