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 other cameras!
But it turns out you can use a lot of third party cameras using the Macam driver.
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.
Barcode Reader!
EvoBarcode will let you use your camera as a bar code reader.
Web Streaming!
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.