How do you get the mildew smell out of towels?

I add about half a litre of vinegar (white, brown, doesn't matter) to a 40-degreewash - don't add anything else, no detergent, no softener. This not only kills and gets rid of the mildew but also makes sure that other chemical residues are removed. Then line-dry. As other posters have said, try adding a cupful of vinegar to every wash as a preventative measure - a thousand times cheaper than Calgon tablets and does exactly the same thing. The slight smell of vinegar completely vanishes in the air as it dries.