V-Ray for Sketchup
V-Ray is a well known and powerful renderer for many other applications and the specific Sketchup version is an excellent choice for Sketchup work.
I've tested many renderers for sketchup: Podium, IrenderNxt, Kerkythea, LIght-up, IDX renditioner as well as rendering in other apps ( Vue 7 and Carrara Pro 5) Each of them has their own peculiarities and ways of doing things that are too involved to outline here. The principle benefit of V-Ray for Sketchup is that it works entirely within Sketchup (as do some of the others mentioned above) so that it's easy to refine the model and textures within Sketchup and re-render without any exporting. The interface is fairly simple at first glance, but has a lot of depth, which takes a while to get the hang of. The resulting renders come out relatively fast and look extemely natural. Many of the others retain a less than photo-real look.
The only negative aspect is that it doesn't support volumetric light.