Use USB webcam instead of built in FaceTime webcam

There are some websites where you can turn on your webcam to chat with other people. On some of these sites it is possible to select which cam you want to use if you have more then just one connected. But on some other sites you can't select which cam you will use. If you visit those sites with an iMac you always get the built-in iSight cam turned on. There seems to be no way on these sites to select any other webcam that is connected via USB.

I already tried the script named "isight disabler" found here: http://techslaves.org/isight-disabler/, but this script turns off ALL cams, not just only the iSight cam.

Is there a way to selectively turn off only the built-in iSight cam, without turning off any other cams?

You could give CamTwist a try. It lets you take video inputs and remap them to virtual video sources that most apps can access.

I’ve used it to remap a Firewire video camera connected to my laptop (via a Firewire to Thunderbolt connector) as my main video chat camera when doing a google hangout. It also let me overlay graphics into the camera video before sending it out to google.