If you can settle without the wavy things and just scroll, num and caps lock keys... heres JUST the one you need:
http://www.msgpluslive.net/scripts/view/78-Key-Flasher/
I reccomend that you only let the scroll lock key flash though, otherwise it gets really nasty when typing
Also when the interval is too low it doesnt work, 120ms seems the cap for my keyboard