Some useful commands...

[Nimy]

top---Command for performance monitoring. It display CPU usage, Memory usage, Swap Memory, Cache Size, Buffer Size, Process PID etc..
htop---Much advanced interactive and real time Linux process monitoring tool.
iotop---Much similar to top command and Htop program, but it has accounting function to monitor and display real time Disk I/O and processes. Useful for finding the exact process and high used disk read/writes of the processes.
vmstat---Command to display statistics of virtual memory, kernerl threads, disks, system processes, I/O blocks, interrupts, CPU activity etc..
netstat---Command line tool for monitoring incoming and outgoing network packets statistics as well as interface statistics.
iostat---Command line tool that will collect and show system input and output storage device statistics. Used to trace storage device performance issues including devices, local disks, remote disks such as NFS.
lsof---Command to display list of all the open files and the processes.
tcpdump---Most widely used command-line network packet analyzer or packets sniffer program that is used capture or filter TCP/IP packets that received or transferred on a specific interface over a network.