The OpenCV (Open Source Computer Vision) works under Linux, Windows and Mac OS X.
I use OpenCV under Mac OS X and it works very well with the built-in iSight of my MacBook, and I believe it works equally well under the other platforms.
Of course, OpenCV is not primarily intended as video capture library, but it can be used as such. And if you want to process the frames you grab from your webcam OpenCV will definitely help you with that...