In reply to Sarah Gooding.

It's not a WordPress plugin (though I've asked about if it can be ported into one), but JPEGmini is a local Windows and Mac app that does amazing things with JPGs, cutting down bytes by up to 70%, with no visual difference. I dropped a 10.7GB directory of mostly JPG files on to it, and it was just 5.72GB when it had finished!

Thanks for the post Sarah - I was wondering if there was something with the power of JPEGmini for PNG, instead of WP-Smushit.