Not all Macintosh computers have built-in iSight web cameras. Older G4 and G5 Macintosh owners, or those with a Mac Pro tower or Mac mini often discover finding a fully Mac compatible web cam to be a challenge. Not all webcams made (especially in the mid to low-end price range) work on a Mac, or even have webcam drivers for anything other than Windows PCs.
Ideally you want to buy a web camera based on a new webcam standard: UVC - "USB 2.0 Video Class" which is supported by OSX 10.5 Leopard - and 10.4.11 Tiger. NO DRIVERS NEEDED, the driver needed is built into OS X - so getting a webcam working is just plug and play easy.
It is important to note that Video Class compliant webcams are based on the USB 2.0 speed standard. So you've GOT to have at least a fairly recent Mac (generally with a processor speed of 1GHZ or faster) to take advantage of a modern driverless webcam. USB 1.1 is just TOO slow to deliver the video chat quality and frame-rates you need for video conferencing.
Beyond iChat and PhotoBooth included with current Macs - other great video chat and capture programs are worth exploring - including Skype For Mac, Yahoo Instant Messenger, ISPQ, CamFrog - and more.
For those wanting to make and upload their own videos for YouTube or elsewhere - Using PhotoBooth or QuickTime Pro's video capture function is simple and straightforward. Lastly, It's important to know that external USB webcams can't be used directly with Apple's iMovie: You've GOT to use some other program to capture the video, then you can bring it into iMovie for final editing. Only FireWire webcams like the original 1394 iSight or select DV camcorders with FireWire can capture directly in iMovie.
