Zamzar - Free conversion online of many type of documents (images, text, video, audio...
FastStone Photo Resizer - Convert and Rename images in batch mode Support JPEG, BMP, GIF, PNG, TIFF and JPEG2000
PDF-XChange Editor - The smallest, fastest, most feature-rich PDF editor/viewer available
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
CloudConvert - convert anything to anything - more than 200 different audio, video, document, ebook, archive, image, spreadsheet and presentation formats supported.
ImageMagick - ImageMagick is a software suite to create, edit, and compose bitmap images.