Nomacs is an interesting cross-platform image viewer with one or two unusual features.
The program opens much like any other image viewer. You can display individual files or complete folders, for instance (there's support for viewing all the main formats, including many RAW file types). A thumbnail browser helps you manually navigate to the images you need, or you can use the Player to create an automatic slideshow.
The viewing area can be customised in various ways. You can choose to view basic details (file name, creation date, rating), metadata, histogram and more. Nomacs can run full screen, if you like, or you can reduce its opacity to leave the program very much in the background.
There are some very basic image editing options, too: "resize", "crop", "rotate" and so on. Nothing even faintly surprising, but they might come in useful occasionally.
What's more unusual, though, is that multiple instances of nomacs can be connected and work together. If you've two copies of nomacs running on the same PC, say, you can synchronise them, so that when you pan or zoom on one instance, the second does exactly the same thing (very useful when you need to compare separate images).
You can do the same thing across a LAN, too, as well as transferring images from one networked computer to another.
Figuring out how to do this can be a challenge, unfortunately, as it's not even faintly obvious, and there's no help provided with the program. We were initially baffled, but fortunately there is a speedy solution: just head off for a quick look at the online help and you'll soon have everything working.