I had to modify Jeroen's code to get the `marks` shortcut working under Mac OS 10.8:

    export MARKPATH=$HOME/.marks
    function jump {
        cd -P $MARKPATH/$1 2> /dev/null || echo "No such mark: $1"
    }
    function mark {
        mkdir -p $MARKPATH; ln -s $(pwd) $MARKPATH/$1
    }
    function unmark {
        rm -i $MARKPATH/$1
    }
    function marks {
        ls -l $MARKPATH | sed 's/  / /g' | cut -d' ' -f9- && echo
    }