OS X: How to perform a lossless crop of a JPEG image?

I use Xee. It is so much better than preview for images (IMHO); especially since you can traverse images within the same folder by clicking the arrow buttons. All of the button short cuts are reprogrammable.

It supports lossless cropping and rotation. Just see the webpage.

If you like it, buy milksnake to help the developer out - I did.

This command worked for me:

jpegtran -crop WxH+X+Y infile.jpg outfile.jpg

While perhaps not lossless I found FFmpeg to also do quite well:

ffmpeg -i infile.jpg -filter crop=W:H:X:Y -qmax 1 outfile.jpg

Based on this answer, I can suggest using the command-line tool jpegtran, the GUI-based Jpegcrop Windows program, or one of the many programs based on IJG jpegtran code (although not all of them support cropping).