I have been happily using neatimage for a while. Here is a little test made on this picture.

Right out of the camera. Normal settings:	Filtered using neatimage:
Too much jpg compression:	Filtered with neatimage:
zoom x2	zoom x2 filtered image:

The noise in the clouds is very well filtered and you are not losing any of the details on the mountains, the pole and the wires.

Neatimage, try it: http://www.neatimage.com. It's neat, and it is free!