Some really interesting work is being done on Cycles by community member tuqueque. One of his results: improved image quality with ‘Importance Sampling’
Josh Faulkner writes:
User MikeFarny has created a patch to Brecht’s Cycles render engine to perform improved sampling for certain types of scenes that have had some very interesting results. The Blenderartists link has the history and some user tests that show massive improvements in image quality – there are generally less samples in the same amount of time compared to vanilla Cycles, but since the samples are smarter, the resultant image is clearer. Check it out!
Three of the more astonishing examples from the thread: