Using a Green Screen with Skype?

I just got a new Logitech webcam and need to make a Skype video call, but need a green screen and a background image.

How do I accomplish this?

I seem to recall install an app a few years ago which acts as the input to Skype, and on which I merge the webcam live video stream with a background in place of the green screen.

If so, please mention a few names of freeware that can do this.

Thanks.