OpManager is a network monitoring software that can automatically discover your network; group your devices into intuitive maps, monitor devices in real-time and alert instantaneously on failure. It serves as a complete network monitoring solution by providing complete fault and performance functionality across your WAN and LAN infrastructure.
monit is a utility for managing and monitoring, processes, files, directories and devices on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.
htop is a very competent interactive process viewer for the shell, providing all the functionality -- and more -- that the top utility does, in an easier and more intuitive way. htop provides easy-to-use menus for most operations and also has support for using a mouse.
saidar is a curses-based application to display system statistics. Statistics include CPU, processes, load, memory,swap, network I/O and disks I/O along with their free space.
ZABBIX is a 24×7 monitoring solution without high cost.ZABBIX is software that monitors numerous parameters of a network and the health and integrity of servers. ZABBIX uses a flexible notification mechanism that allows users to configure e-mail based alerts for virtually any event. This allows a fast reaction to server problems. ZABBIX offers excellent reporting and data visualisation features based on the stored data. This makes ZABBIX ideal for capacity planning.
Darkstat is a network statistics gatherer.Effectively, it's a packet sniffer which runs as a background process on a cable/DSL router, gathers all sorts of useless but interesting statistics,and serves them over HTTP.