Listening for Keystrokes

Hi,
I'm an Arduino newbie, i use a mac and I also need help with listening for keystrokes and sending them to Arduino via USB. Ideally, i would like this to work regardless of which program I am actually typing in. I guess i need something like a keystroke logger running in the background and sending keystrokes to the serial port, but I was wondering what my simplest option is.
Many thanks!

What is listening?

What is sending?

Your description is not much help. I have no idea what you want to do.