arpwatch is an open-source software that monitors a computer network for ARP-activity. It generates a log of IP address-MAC address pairings along with a timestamp when the pairing appeared on the network. The first and foremost reason to monitor ARP activity is to detect ARP spoofing. It is developed by ...
lsof is a Unix-specific diagnostic tool. Its name stands for LiSt Open Files, and it does just that. It lists information about any files that are open by processes currently running on the system. It can also list communications open by each process.
Du (disk usage) reports the disk space usage for the directory you specify. By default it recurses directories to show the total size of a directory and its subdirectories.