ImageOptim - Faster web pages and apps.
PhotoPenguin App - PhotoPenguin is a completely free batch photo editor with support for cropping, resizing, adding watermarks, and converting photos to other formats. It works on the user's device, using the capabilities of modern browsers.
JPEGmini - JPEGmini - The Photo Optimization Tool Trusted by Tens of Thousands Image Perfectionists
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats