TinyPNG - Make your website faster and save bandwidth. TinyPNG optimizes your PNG images by 50-80% while preserving full transparency!
Online Convert - Convert files like images, video, documents, audio and more to other formats with this free and fast online converter.
ImageOptim - Faster web pages and apps.
Convertio - File Conversion in the Cloud
Oxipng - Oxipng is a multithreaded lossless PNG compression optimizer. It can be used via a command-line interface or as a library in other Rust programs.
IMGCentury.com - This is free online image compressor you can use to compress image size. You can easily reduce multiple images sizes by using online image compressor tool.