I sort of want the equivalent of cd - for git. If I am in branch master and I checkout foo, I would love to be able to type something like git checkout - to go back to master, and be able to type it again to return to foo.

Does anything like this exist? Would it be hard to implement?

From the release notes for 1.6.2

@{-1} is a way to refer to the last branch you were on. This is
accepted not only where an object name is expected, but anywhere a branch name is expected and acts as if you typed the branch name.
E.g. git branch --track mybranch @{-1}, git merge @{-1}, and
git rev-parse --symbolic-full-name @{-1} would work as expected.

and

git checkout - is a shorthand for git checkout @{-1}.

To see the list of previous checkouts:

i=0; while [ $? -eq 0 ]; do i=$((i+1)); echo -n "$i. "; git rev-parse --symbolic-full-name @{-$i} 2> /dev/null; done

This Bash one-liner script is not perfect but it should work for most cases. Note that sometimes the number may skip.

Tip: You can add it to .bashrc as a function.