(Okay, I lied, there are slightly more steps involved.)
- Render the picture in blender. In reasonable time, accepting the fact that there will still be some noise around.
- Open the finished image in Photoshop CC.
Filter > Camera Raw Filter.
- Go to the Tab “Detail”. Crank up the slider for Luminance Noise Reduction.
- See the magic happen.
Of course this should work with other path tracing renderers. It works because apparently Adobe developed a pretty good noise reduction algorithm for its Camera Raw plugin, which is used in Lightroom (so you could do that improvement in there as well) and since CC (at least, maybe even CS6) also in Photoshop through the Camera Raw Filter.