Zabbix is the ultimate open source availability and performance monitoring solution. Zabbix offers advanced monitoring, alerting, and visualization features today which are missing in other monitoring systems, even some of the best commercial ones. Below is a short list of features available in Zabbix:

• auto-discovery of servers and network devices
• low-level discovery
• distributed monitoring with centralized web administration
• support for both polling and trapping mechanisms
• server software for Linux, Solaris, HP-UX, AIX, FreeBSD, OpenBSD, OS X
• native high performance agents (client software for Linux, Solaris, HP-UX, AIX, FreeBSD, OpenBSD, OS X, Tru64/OSF1, Windows NT4.0, Windows 2000, Windows 2003, Windows XP, Windows Vista)
• agent-less monitoring
• secure user authentication
• flexible user permissions
• web-based interface
• flexible e-mail notification of predefined events
• high-level (business) view of monitored resources
• audit log

Source: http://www.zabbix.com/index.php