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