You may want to try iGlasses. From the website:
Adjust and manipulate your webcam's video settings from within iChat and many other programs, including iMovie, Photo Booth, Yahoo! Messenger, Skype and web-based chat. Apply easy preset settings and fun effects to your video conferences and recordings.
Adjust colors, brightness, and even pan and zoom your Mac's built-in iSight. The picture from your iMac, MacBook or MacBook Pro has never looked so good.
Is your external iSight looking a bit dark? You could install studio lighting, or just install iGlasses to give your videos an instant brightness boost.
iGlasses is a great companion to your iMage USB Webcam. Fine tune your colors for a perfect picture every time.
It's not just for iSight! If you use a Logitech QuickCam or other 3rd-party webcam that uses UVC or the macam driver, iGlasses will add the same great adjustments and features.