I learned of a cool feature in the cd command, which to my knowledge works under zsh, bash, ksh and possibly other shells.
cd - takes you to your previous directory, letting you easily switch back and forth between two directories. Take /home/ryan and /etc for example:
ryan@lambda:~$ cd /etc ryan@lambda:/etc$ cd - /home/ryan ryan@lambda:~$ cd - /etc ryan@lambda:/etc$
Please share any other cool shell tricks you may know :D