I was going to mention Knusperli as well. It's worth mentioning that the "no more artifacts" claim in the title depends heavily on what you understand to be an

I was going to mention Knusperli as well. It's worth mentioning that the "no more artifacts" claim in the title depends heavily on what you understand to be an artifact. With modern, good quality JPEG encoders (e.g. mozjpeg), blocking only appears when the file is heavily compressed, or after a lot of generation loss. A decoder like this can't recover information that isn't there, obviously. So at more moderate levels of compression, where there's no noticeable blocking but the information inside the blocks is distorted or missing, it's not likely to help you a lot. (As a citation, I can only offer the fact that I've frequently tried to clean up JPEG images I've found online, and Knusperli is usually my first step. It's never a sufficient step, though.)