Version 9 of the libjpeg library from the Independent JPEG Group has been released. This version of the JPEG library is said to noticeably improve the lossless JPEG compression support even to the point that libjpeg can now output compressed lossless JPEGs of smaller size than PNG images.
The new lossless coding mode for libjpeg 9 is unfortunately not backwards compatible with earlier releases of this JPEG library. The compression abilities are increased significantly with IJG's libjpeg 9 but with included color transform, the compressed JPEGs from this library will break with older JPEG decoders.
The version nine release notes state about this new support, "Add cjpeg -rgb1 option to create an RGB JPEG file, and insert a simple reversible color transform into the processing which significantly improves the compression. The recommended command for lossless coding of RGB images is now cjpeg -rgb1 -block 1 -arithmetic."
Aside from the significant compression improvements, there's also other changes and enhancements to this widely-used compression library as noted at Infai.org