Virtually any modern webcam will work with a Mac through the generic driver.
However, what you won't get is an optimised webcam like you would for Windows. For example, last year I bought the Creative Livecam Socialize from OCUK, it's marketed as being Mac compatible.
Mac compatibility basically just means that you can plug it in and it will work, which it does. However, when I ran it on Windows with the drivers and included software it had a lot more functionality. Firstly the aspect ratio changed a little, the frame rate is smoother, the button on top of it will take a picture. There was also a number of things you could do with included software, from making silly faces to creating a security cam and controlling aspects like exposure.
Really I think you need to buy one that is marketed as Mac specific, I believe Logitech did one when I was looking, but it was expensive. There is Apple's old iSight available secondhand, but that's getting on a bit now.
What I'm using is fine for Skype and such. But if you were making videos for YouTube or something the quality isn't as good HD quality as it is under Windows.