Image Optimisers
TinyJPG | TinyPNG
Online tool to compress images. Drag and drop interface
tinyjpg.com
free
Compressor.io
Compress your images online. 10MB file size limit per file.
compressor.io
free
ImageOptimizer
Free service to compress and optimize your images
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free
Smush.it
WordPress plugin to optimize your images
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free & paid options