We've been using http://www.jpegmini.com/ to compress JPGs for our apps. Worked OK, although we didn't get the enormous reductions they advertise. However 5% - 10% does still make a difference.

We've been using the desktop version. Would love to use something similar on a server, but jpegmini is overpriced for our scenario (I'll not have a dedicated AWS instance running for compressing images every second day or so). Will definitely check out this project :)