I learned this one recently from a coworker:

cd -

This will allow you to switch to your previous current working directory. So:

# Go into first directory
cd ~/path/to/first/dir
# Go into second directory
cd ~/path/to/second/dir
# This takes us back to the first directory
cd -
# This takes us back to the second directory
cd -

This is really useful if you have two main directories you're currently working in.