From BetterJPEG Team:
Better JPEG Lossless Resave plug-in for Adobe Photoshop is a tool designed to avoid recompression loss when editing and resaving JPEG images in Photoshop. The plug-in takes advantage of the fact that JPEG images consist of a number of small independent blocks and does not recompress blocks that don't have to be changed.
The plug-in consists of two parts: Better JPEG Lossless Import and Better JPEG Lossless Export.
This plug-in is mostly helpfull when only a part of a JPEG image requires editing and it is important to protect other parts of the image from degradation, in other words it can be used for such image modifications as adding text, date stamps, watermarks, or removing/correcting objects (birthmarks, red eyes, pipelines, garbage, date stamps, etc.).