SmoothDraw Alternative to Make Videos like Khan Academy
Try Krita. It works on Mac, Windows, and Linux. It's designed more for painting, but it should work just fine for this.

I believe it works with tablets, although I'm not 100% sure. It doesn't have an infinite canvas (if you even need that), but you can always start with a canvas bigger than the screen and use it like a whiteboard. Middle-clicking-and-holding will scroll the canvas. (If you don't have a three-button mouse, hold Space and drag. Two-finger scrolling will not work.)

Plus, it's free and open source!



If you do need an infinite canvas, try Inkscape. Unlike Krita, it uses vectors, so you can zoom in and out without worrying about quality. I don't think it supports graphics tablets, though.

If you go this route, when you launch Inkscape it will ask you to install X11. Be aware that it won't work quite like other Mac apps—it's running in its own windowing system. One main difference is that Ctrl will work as an alternate Cmd key.