For Macs
Most new Macs come with built-in webcams. If you have an older mac, there is really only one easy solution: the Apple iSight. The iSight is the gold standard for webcams, and while you'll pay a premium for it, you won't regret it.

While your Mac may have a USB port available, most USB webcams designed for PCs won't work on your Mac without special software installed.

Recently, Apple discontinued the iSight!

You can try finding a used iSight here or on eBay, or try downloading "macam" ( http://webcam-osx.sourceforge.net ), software that enables to use many USB webcams with your Mac.
Apple iSight FireWire Webcam for Apple Computers M8817LL/A
Apple iSight FireWire Webcam for Apple Comp...