OS X is capable of using USB and Firewire cameras, with perhaps the most famous being the iSight, second to the Built-in iSight of Apple’s laptops.
Use multiple cameras!
If you’re using multiple cameras at once, say for security monitoring, you’re going to want to take a look at SecuritySpy, which has motion detection, time lapse, and the ability to view from a remote browser, plus many other features.
EvoBarcode will let you use your camera as a bar code reader.
EvoCam will let you stream multiple cameras and push images to servers.
Stop Motion Photographer!
iStopMotion will help you make your own stop motion movies.
Real-time Special Effects!
iGlasses will enhance and alter your video feeds.