How can I undo the last cd command?

You can use cd - to go back to your previous location, no matter where that was.

cd - is great for going back one level, but if you find yourself wanting to go back a few levels, check out this script:

acd_func.sh

It's great. cd -- to see your history, and cd -3 to go back 3 levels, for example.

You can also use pushd and popd to utilize the directory stack :)

Some info here