IrfanView is probably the most popular and feature packed free image viewer for Windows. It has been around for many years and keeps getting better. In addition to standard image viewing, it includes many additional features including slideshow, batch conversion, image adjustment, image editing, resizing, panorama stitching, auto EXIF rotation and more. The functionality can be further extended via plug-ins that are widely available. There may be more user-friendly and attractive image viewers, but few that offer all the functionality, and extendibility that IrfanView can provide.