One on my favorites and most useful has to be the search functionality. To use this simply hold down CTRL + R in your Terminal or Command Prompt and type in a character and you will see entries from your history, keep entering characters to narrow down your search.
Human readable sizes
ls with the -lh option, shows sizes in human readable format.
$ls -lh
Clear current window
Simple way to clear your current window, use this to make visualizing terminal easier.
$clear
Show hidden files
When using list you can add a -a to show hidden files and folders.
$ls -a
