nomacs is described as 'small, fast image viewer 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' and is a very popular Image Viewer in the Photos & Graphics category. There are more than 50 alternatives to nomacs for a variety of platforms, including Windows, Linux, Mac, Xfce and iPad. Most alternatives are Image Viewers and Image Editors. The best alternative is IrfanView. It's not free, so if you're looking for a free alternative, you could try ImageGlass or JPEGView. Other great apps like nomacs are FastStone Image Viewer (Free Personal), XnView MP (Free Personal), XnView (Free Personal) and qView (Free, Open Source).
ImageGlass is a simple picture viewer with which you will be able to display your image collection in a very similar way to the one that Windows provides by default, but with some additional functions.
FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor. It has a nice array of features that include image viewing, management, comparison, red-eye removal, emailing, resizing, cropping and color adjustments.
ACDSee is a complex, multi-purpose application that serves for viewing, organizing and editing digital photos. Furthermore, images can be shared on social websites or your own space on the developer’s website.