Here are some simple but most used ones.

  # go back to the last directory
  cd -

  # run last command with sudo
  sudo !!

  # truncate file
  > /tmp/file-to-truncate.txt

  # show directory size
  du -sh my-dir

  # refresh output of command every 2 seconds (needs watch command)
  watch ls /tmp

  # check how long a command takes
  time du -sh mydir

Then I also have a neat function for cd ..:

  function ..() { 
    for i in $(seq 1 $1); do cd ..; done 
  }

  # cd ..
  .. 

  # cd ../../../../
  .. 4