Nagios
Provides enterprise-class Open Source IT monitoring
Nagios provides complete monitoring of desktop and server operating systems – including system metrics, service states, process states, performance counters, event logs, applications (IIS, Exchange, Apache, MySQL, etc), and services (Active Directory, DHCP, Sendmail, etc). Nagios supports monitoring of Windows, Linux, Unix, Solaris, AIX, HP-UX, and Mac OS/X operating systems.
Implementing effective operating system monitoring with Nagios offers the following benefits: