ImageOptim is a front-end (GUI) for set of tools for optimisation of PNG and JPEG images.
Optimisation makes files smaller (in terms of disk space) by finding optimal compression parameters and by removing unneccessary information, like file comments, EXIF tags and color profiles.
NONTROPPO This is a wonderful tool; simple to use yet with a whole set of different optimization options available. Drop your images and let it do its magic. It gets a precious space in my dock... (Version 1.0.1)
ARVIDTP Great little app. Sometimes reduces the filesize of small PNGs saved from Preview by about 45%! Whenever I develop a wordpress theme, I just drop the folder on this before uploading to the web - couldn't be simpler. (Version 1.0.10)
PORNEL It may do that only when it doesn't degrade quality of the image (i.e. if you have low-color image unnecessarily saved as 24-bit). (Version 1.0.2)