Nagios is an excellent choice if you want to perform any sort of monitoring. Nagios’ main strengths are:
Robust and Reliable
Nagios runs on many Operating System
Nagios can be used to monitor all sorts of things, here are some common things are typically monitored:
Ping to see if host is reachable
Services such as DHCP, DNS, FTP, SSH, Telnet, HTTP, NTP, POP3, IMAP, SMTP etc.
Database servers such as MySQL, Postgres, Oracle, SQL Server etc.
Application level information (Apache, Postfix, LDAP, Citrix etc.)