Crunch Awesome PNG Image Optimizer (github.com)

Keep in mind that this is lossy compression.

  The following examples demonstrate the benefits and
  disadvantages of the current iteration of this aggressive
  space saving lossy transformation strategy. In many cases,
  optimization with Crunch will lead to excellent space
  savings and an imperceptible decrease in image quality.

Does it have to be specific to OSX, would it be hard to make it available on Linux as well?

Those examples are stunning!

There is a ticket explaining where it works well and where it doesn't:

https://github.com/chrissimpkins/Crunch/issues/15