PunyPNG.com is a free online Image compression tool for developers and bloggers. PunnyPng is used by Wikipedia, Ask.com and Dictionary.com. They claims for the best compression as compared to other services like smush.it, OptiPNG and ImageOptim.
I also uses sometimes this services and find it good. Sometimes it works better than other freeware including smush.it, but sometimes it fails. The failure count is significantly low and so you may rely on the service. They allow free users to compress image of size 150KB and maximum 15 images, which is quite low. Due to this limitation users stays away from it. If you use this type of services rarely, You may bookmark this site. You can compress the three most popular image extensions: JPG, GIF and PNG using their services.
Their is a developer option and it is free for registered users. The registrations are free. If you really love their services and wants to increase the limit. You may donate to get some higher quota and get upto 75 files of maximum 500 KB. Their is a small support forum to discuss and for making request for features.
PunnyPNG is a better service for image compression without loosing quality. but due to the limitation they have, they got very less traffic and are less popular.
I think the paid quota is still small for some users and should be increased. If you really aware about size and number go for OptiPNG (This is also integrated in Google PageSpeed plugin) or smush it.