drawing with animation

Nothing built into Keynote would do that. You could record a QuickTime movie of a circle being drawn. I think Keynote will play animated gif files but I'm not positive of that.

If you're particularly clever, and your circle is on a solid background, you might be able to use a shape coloured like the background to cover the circle, then use an Action such as Move or Rotate to reveal the circle outline as if it were being drawn. I'm not positive you would get the results you want, however.

That's a clever approach, ecnelis. One could do a similar thing directly in Keynote (again, if your background is solid). Create your circle, then create four separate objects as masks, one covering each quadrant of the circle. Fill the masks so that they are the same colour as the background, so that the circle disappears. Now apply a Wipe Build Out to each object, using the appropriate directions of Wipe, so that as they disappear the circle is revealed. This should get you very close to the effect you want (and if you create quarter-circle masks, the result will be nearly perfect).

Another alternative -- if your circle is in the centre of the slide, you can use a Radial Transition between a blank slide and the circle slide (you can, of course, have other elements on both slides, so that the transition only reveals the circle, and doesn't appear to be an actual transition between slides).

Darn! That’s good!