IrfanView is a fast and compact image viewer/converter.
It is trying to be simple for beginners and powerful for professionals.
Many supported file formats and features.
Features include:

multi-language support, Thumbnail option, Painting, slideshow, toolbar skins, fast directory browsing, batch conversion/editing, multipage editing, file search, change color depth, scanning, cut/crop, IPTC edit, capturing, lossless JPG operations, effects, ICC support, EXE/SCR creating, many hotkeys, command line options and plugins.