Nomacs
Nomacs is an open-source image viewer, editor and synchronizing software that allows viewing all common image formats including RAW and .psd images and supports multiple image platforms. It includes image operation methods for adjusting brightness, contrast, saturation, hue, gamma and exposure. One can crop images, resize them, use pseudo color functions, export images to TIFF mufti-page type, create mosaics, etc. Nomacs also helps you perform synchronizing functions like synchronized zooming, synchronized panning, synchronized next/previous file, overlay of two or more instances (with changing opacity), arrange synchronized instances, synchronize over LAN, etc. Nomacs enables you to browse images in zip or MS Office files which can be extracted to a directory. Nomacs is licensed under the GNU General Public License v3 and is available for Windows, Linux, FreeBSD, Mac, and OS/2. It is free for private and commercial use. 

Why you must get it: 
Additional user-friendly settings are available on Nomacs such as metadata, thumbnail previews, cache, etc. Metadata stored with the image can be viewed and you can add notes to images that can be edited, which may or may not be opaquely displayed to the user's needs. A thumbnail preview of the current folder is included as well as a file explorer panel which allows switching between folders. Within a directory, you can apply a file filter, so that only images are displayed whose filenames have a certain string or match a regular