I'm looking for a simple way to losslessly crop JPEGs on Mac OS X.
Particularly, is there some way to get Preview to perform a lossless crop?
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.
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).
I you are worried about "lossless" - don't use jpg. Every time you open one, and then save it - you are losing some data - even at the highest setting. If anyone disputes this, just open an untouched image in PS (for instance), and then save it and open it several times at the highest setting. You will notice artifacts. (Magnify the file several times and compare with the original.)
So just crop it and save it in preview. There will be almost imperceivable loss.