Sort By Memory Percent Usage With Top

Use the top command in Linux/Unix:

top

press shift+m after running the top command
or you can interactively choose which column to sort on
    press Shift+f to enter the interactive menu
    press the up or down arrow until the %MEM choice is highlighted
    press s to select %MEM choice
    press enter to save your selection
    press q to exit the interactive menu

Or specify the sort order on the command line

on OS-X

top -o MEM

other distros

top -o %MEM