WhoisUP monitors hosts via ICMP and shows the status in a webpage. When a host is down, it can send mail, write a syslog message, open a popup message, or play a sound. It uses Nmap to check hosts continuously.
The Deimos Project is a computer statistics/monitoring tool allowing you to gather information and statistics about your computers, including network information and bandwidth, uptime information, keyboard and mouse usage, and hardware information.