top – When you want an overview of all the processes or threads running in the system: top is a good tool
htop – Htop is essentially an enhanced version of top. It’s easier to sort by processes.
apachetop – Apachetop monitors the overall performance of your apache webserver.
powertop – helps you diagnose issues that has to do with power consumption and power management
iotop – checks the I/O usage information and gives you a top-like interface to that.
iftop – iftop is similar to top, but instead of mainly checking for cpu usage it listens to network traffic
ethtool – used for displaying and modifying some parameters of the network interface controllers
iptraf – iptraf gathers a variety of metrics such as TCP connection packet and byte count, interface statistics and activity indicators,
bmon – Bmon monitors and helps you debug networks.It captures network related statistics and presents it in human friendly way.
mtr – MTR combines the functionality of traceroute and the ping tool into a single network diagnostic tool.
justniffer – Justniffer is a tcp packet sniffer. You can choose whether you would like to collect low-level data or high-level data with this sniffer.
dstat – Dstat aims to be a replacement for vmstat, iostat, netstat and ifstat. It allows you to view all of your system resources in real-time
lsof – Lsof stands for ls open files, which will list all the open files in the system.