Screen space ambient occlusion provides a rough approximation of real-time global illumination. It is fairly inexpensive to render and will run easily on shader model 3.0 cards and up. The difference in image quality is obvious between the SSAO-lit image and the image with direct lighting only.
Video here:
http://www.youtube.com/watch?v=j_98JDOmuhw
Video here:
http://www.youtube.com/watch?v=j_98JDOmuhw