Ambient Occlusion Material
To make an ambient occlusion pass (or at least something close), I color everthing in the scene pure white with a material that has the diffuse, specularity, and ambient properties turned to zero, and the Occluded Ambient channel set to something like .9 . Render in best mode. You don't even need to have any lights on in the scene.
Not perfect, but it works nicely for darkening edges and depressions. I then multiply this pass on top of the color, adjusting levels for best look.
I also do occllusion passes with a white fast shader and several lights coming from every direction. Check out the Ambient Occlusion Light Setup
. Each lights have shadows, only the depressed areas keep the shadow (because the light fills in everywhere else). I sometimes use both, or none...