[chrismeller]

Surprised no one has mentioned Yahoo!'s smushit.com service yet. It even works nearly identically (minus the API).

Kraken seems to do a tad better in the handful of images I handed it (all PNG sprites), but not remarkably so.

The Google PageSpeed extension for Chrome will also give you optimized images as one of the steps, if it thinks there is room for improvement.

[tekacs]

There is a comparison to smushit right there on the homepage. :P

[chrismeller]

Yes, but no one has mentioned it and the differences in my tests were far less drastic than the comparison would suggest.