|TinyPNG reduced this png screenshot from 94KB to 27KB. That's a 70% file size reduction.|
TingPng.org is an online tool that reduces file sizes of PNG's while maintaining transparency if there is any. The service is free and support files up to 2MB. Its the best PNG optimization and compression service out there and generally reduces the files size of my images by 15 to 75 %. The service supports up to 20 image conversions at a time.
PunnyPNG is another service out there that is popular and used by companies such as Wikipedia, Dictionary.com, Ask.com etc. Unfortunately, its free service offers png image compression that are limited to 100kb. That expands to 500kb when you sign up for its $2 premium web service. The png file size reduction are on par with that of TinyPng. However. PunnyPNG does offer advanced image compression that extends to JPEG format with its premium service.
There are also desktop tools like ImageAlpha that offer advanced png file optimization and reduction. As of now I recommend TinyPng for all your png file compression needs. It reduces file formats that are as big as 2MB, its an easy drag and drop service and best of all, its free !