Radical Image Optimization Tool (RIOT) is a free image optimizer that will let you to visually adjust compression parameters.

PNGGauntlet is a PNG compression app for Windows.

OptiPNG is a PNG optimizer that recompresses image files to a smaller size, without losing any information.

File Converter is a very simple tool which allows you to convert and compress one or several files using the context menu in windows explorer.

NessViewer is a software to edit, collect, present and stream media.

ImageOptim makes images load faster. Removes bloated metadata. Saves disk space & bandwidth by compressing images without losing quality.

JPEGmini is a photo optimization software.

Light Image Resizer can resize and convert images one by one or in batch as well as other features.

Caesium is an image compression tool.