Clarification (thanks @vgm64): if you're already in Terminal, this lets you quickly change to the topmost Finder window without leaving Terminal. This way, you can avoid using the mouse.
I've added the following to my .bash_profile
so I can type cdff
in Terminal at any time.
function ff { osascript -e 'tell application "Finder"'\
-e "if (${1-1} <= (count Finder windows)) then"\
-e "get POSIX path of (target of window ${1-1} as alias)"\
-e 'else' -e 'get POSIX path of (desktop as alias)'\
-e 'end if' -e 'end tell'; };\
function cdff { cd "`ff $@`"; };
This is from this macosxhints.com Terminal hint.