SketchUp add-ons and plugins from Render Plus Systems and others.
IRender nXt review
sketchuppluginreviews.com has posted a review of IRender nXt:
"The nXt Renderer by IRender is a rendering plugin that integrates really well with SketchUp. The learning curve on this plugin is very small and you can start rendering almost immediately after installing the plugin. The render quality is pretty good..."