ImageOptim is a must have as part of your responsive workflow when dealing with images. The Mac App, and now Web Service API, removes metadata from your images and compresses them so that there is no image quality sacrificed.
The image on this page is a perfect example. I head over to the ImageOptim site where I took a screenshot of one of their own screenshots — meta, I know.
After changing it from a PNG screenshot to JPG and reducing the image width down, I saved it out of Photoshop at 70% quality. Then I simply dragged the image onto the app and it compressed it a further 4.3%. Not much, but every bit counts (and often images will save 50% of their previous file size).
Now it has been released as a Web Service with prices starting as low at $9 per month for 1000 optimisations… you’re likely to save more money on bandwidth by using the tool.