I’ve been using this trick for years but it seems it’s never really been blogged about. When saving JPEG files using the “Save for Web” feature (“Save for web & devices” in CS3), the quality slider exhibits some strange behavior. Increasing the quality setting by a single percent usually adds only a little extra file size (about half a kB for the test file I used). Going from 50 to 51 with the same file added a full 10 kB. There’s also quite a difference in image quality between 50 and 51. In fact I feel 51 looks pretty good and is the optimal setting for most web projects. Anything above that is a waste of bandwidth, except perhaps for photography portfolios and such.
So the slider appears to have a little ‘speed bump’ in the middle. Anything up to and including 50 will definitely get you visible artifacts and mediocre image quality at best, anything above 50 looks OK, with 51 being an ideal trade-off between file size and quality. It is definitely true that tweaking these settings for each individual output file will get you the best possible results, but for batches, droplets and brainless image saving, JPEG 51 works better than any other setting.
(Thanks to Anko Elzes for discovering this and telling me about it)