PunyPNG : PunyPNG is a free website optimization tool that dramatically reduces the file size of your images without any loss of quality.
Trimage (lossless) image compressor : Trimage is a cross-platform GUI and command-line interface to optimize image files via optipng, pngcrush, advpng and jpegoptim
Squoosh : Squoosh is an image compression web app that allows you to dive into the advanced options provided by various image compressors.
RegPack : RegPack is a packer intended for use on minified Javascript code.
It is mostly suited to small size constraints (1kb, up to 4kb).
Caire : Caire is a content aware image resize library
Crunch is a tool for lossy PNG image file optimization. It combines selective bit depth, color type, and color palette reduction with zopfli DEFLATE compression algorithm encoding using the pngquant and zopflipng PNG optimization tools.