There are some pretty good tools which allow you to easily optimize your images, without losing quality.
1. tinypng.org – Another quick tool for PNG images for compressing the image to best level without compromising on quality.
2. kraken.io – If you need an online application to compress image formats like JPEG, PNG, GIF, SVG to a point where the image doesn’t loose on quality, but the size comes down by really good amount. I tried with an already compressed image using a desktop application, Kraken compress that file to an extra 10% without losing on quality. Firefox and Chrome extensions are also available for this application.
3. imageoptimizer.net – You can upload the image from your computer and give the image quality and size that you want after processing. It gives you some control (very limited) on the output and compression ratio.
4. punypng.com – To compress PNG files, you can try this online tool. Upload the image (PNG only) from local computer and get the compress version of the image.
5. jpegmini.com – To compress JPEG images up to 5X, you can checkout this tool. There is a up-loader to upload the image from local computer and process the same.
6. compressjpeg.com – For more control on the last result’s quality, I would suggest you to use this application. Once you upload the image from computer, a slider appears to auto-control the quality of the last image. There is a preview panel showing the changes according to the quality slider change, and there you can see the final image quality before downloading the image. There is a zoom-in/out feature also available in preview where one can see the quality of the image pixel by pixel. Application is available on different domains for JPG and PNG images.
7. smush.it – It is a very popular online application to compress images. There is an up-loader as well as URL box to provide the image. There is no way to enter the output quality, the application auto-optimize the image by compressing the same to a limit.