Best webcam to use with Skype and a Mac mini?
Most modern USB webcams support a generic interface called UVC (Universal Video Class). OSX supports most UVC drivers out-of-the-box (true for me in Snow Leopard) so sometimes there aren't even any third-party drivers to install. (In fact, most third-party webcam "drivers" only install useless add-ons that bloat your system.)

Logitech is very good at providing compliant UVC devices, so they're a good starting point.

I have a Logitech C525, and it works great at 720p with its built-in microphone, using both Skype and Facetime. No additional drivers required.. plug-and-play!

Older webcams that do not support UVC will require drivers of course. Furthermore, super-new UVC devices may require an extension that OSX doesn't yet handle, obviously drivers are inevitable.

If you want a list of UVC-supported devices, see the Linux UVC page. (Yes, I know Linux != OSX, but this list can be used as a starting point of other brands you can try.)