ACDSee Free is a fast, easy-to-use image viewer.
The program can view all the main image file formats (BMP, GIF, JPEG, PNG, TGA, TIFF, WBMP, PCX, PIC, WMF, EMF). In a click or two you can associate it with those file types so it displays them if you double-click a file in Explorer, or you can just click File > Open within the program to view a file directly.
ACDSee Free provides all the usual image viewing options: there are lots of zoom controls, you can rotate an image, set it as your wallpaper, delete it, perhaps save the file as another format (BMP, GIF, JPEG, PCX, PNG, TGA, TIFF and WBMP exports are supported).
And printing is particularly strong. You can print an image to a standard size (4"x6", 5"x7", 10"x8", whatever you like); there's a choice of filters available if resampling is necessary; you can tweak image gamma, add header and footer text, control the image position, rotation, aspect ratio and more.
ACDSee Free is still a relatively basic viewer. It doesn't provide thumbnail browsing, for instance, and there are plenty of tools around which support many more file formats.
The program is fast, configurable and easy to use, though, and if your image viewing requirements are straightforward then this could be a very good choice.
ACDSee Free isn't overloaded with features, but it carries out the image viewing basics very well, being fast, reliable and easy to use.