-List files opened by a program listening on a certain port (lsof -i:port)

lsof -i:3306

-Benchmark the performance of a webserver

ab -n 100 -c 5 http://www.sample-mysite.com/test.php

Send 100 request with 5 concurrent connections.

-Search for a string/pattern in all subdirectories of the given path

 find /my/path -exec grep 'hairy' {} /dev/null \;