ImageOptim is a small free utility for Mac OS X that does one thing well. It compresses image files but without reducing image quality. On test files I have found that compression rates vary between a few percent up to over 50% and the resulting images look exactly the same.
ImageOptim is only available through the developers web site and NOT the App Store (although the author does note that copies have been sold through the app store so buyer beware).
If you are looking for a quick convenient app to reduce image file sizes without sacrificing original image quality ImageOptim is a recommended download.