JPEGrescan is a perl script that uses jpeg tools to optimize jpeg compression by micro-managing some of the compression math based on research into some of the most common parameters.

$ jpegrescan in.jpg out.jpg

• -s: Removes all Exif data and now all JFIF data as well. (A basic 18-byte JFIF segment is added in its place.)
• -i: Allows optimizations that may be "incompatible" with some software. Currently this means removing all JFIF data (saving 18 bytes) and allowing an encoding not supported by Opera before version 11.61.
• -t: Turns on multithreaded operation. Usually, uses up to 4 threads. Faster, but not four times faster than without -t. So try xargs -n1 -P to shrink a large number of jpegs at the same time.
• -a: Turns on arithmetic coding. (Unsupported by most software.)
• -v: verbose output
• -q: supress all output

First, thanks to Loren Merritt who created this script originally. Also, thanks to the people on devshed.com and lyncd.com - whose names seem to be lost to the sands of time - who came up with the jfifremove idea and basic C code.