nomacs is a free open source image viewer for Windows, small, fast and able to handle the most common image formats including RAW images.
Additionally it is possible to synchronize multiple viewers. A synchronization of viewers running on the same computer or via LAN is possible. It allows to compare images and spot the differences (e.g. schemes of architects to show the progress).
supports the most common image formats including RAW
fast thumbnail preview
zoomable grid preview of thumbnails
display exif information
change transparency of windows
display overview/histogram/file info
pseudo color function
color adjustments (brightness/contrast/saturation …)