Particularly prevalent since Windows Vista, as far as I can see, the 'loading circle' is now an oft-used representation of something being loaded, or some unspecified wait period.
There are many more examples on Google Images. I suppose Apple's age-old 'beach ball' is another example, and one that does use colour - also actually the only filled example of which I am aware.
I'm curious as to the origins of this representation; when it replaced the progressively loading bar (which actually serves more purpose, since it shows distance to completion) as the primary (this is speculative of course, but I think I see more continuous circles than progressive bars these days, especially if you exclude Windows' installers).
I wonder how much analysis has been done user reaction to the representation, the directionality, gradient (often greyscale as above, but Vista was blue), etc.