There are some USB webcams that will work with OS X, such as Logitech’s lineup — you just have to download the OS X drivers from the Logitech website. Then, after you have the drivers installed, you go into the Skype preferences panel, and click on the video tab. There should be a way to change your default camera to the one you have just installed.

Unfortunately, not all webcam manufacturers have released Mac drivers, because it’s still a pretty small market, and a good portion of Macs these days come with a built-in iSight. If your USB webcam’s manufacturer does not support the Mac, then you’ll have to consider buying a new camera with Mac support…and if you do have the option to find an actual iSight, it’s definitely worth the cost, because the picture is fantastic.