XnView is a fast image browser that supports more than 400 graphic file formats, including standard ones such as GIF, JPEG, PNG, BMP, and TIFF, and a myriad of rare ones. (A version of the program for handheld devices, XnView Pocket, doesn't offer as many features, and supports about 15 input formats.) When I encounter a file with a strange extension, I'm almost guaranteed to be able to open and view it. XnView also opens Adobe PhotoShop and Illustrator files, as well as AutoCAD files; CorelDRAW files; the GNU Image Manipulation Program (the GIMP) brushes, icons, and patterns; Kodak proprietary files; several Corel Paint Shop Pro formats; Spectrum 512 formats; several Ulead formats; cursor and icon files; and even audio formats such as AIF, WAV, AVI, VFW, and MPG. XnView even has a hex-view mode, in case you want to see what is actually in a file.
I've tested a number of the formats, and I have no reason to complain. The high image quality beats most commercial programs. Even on a relatively slow machine, XnView is fast, which is especially helpful when you're dealing with large files. The speed of reading and writing depends on the file size, but it appears that CPU-intensive operations such as batch renaming or conversion are optimized for speed.
XnView runs on Windows, Linux, Mac OS X, several flavors of BSD, Solaris, and more. The Windows version offers the most features, while the versions for the other platforms currently miss some functionality, including some file formats that don't exist for non-Windows platforms. XnView is free for personal and non-commercial use, but it's not open source. You can use the associated free GFL SDK library for image manipulation, but you need a special agreement if you're going to use the product for commercial purposes.
Manipulating images in XnView
XnView allows you to choose several modes and layouts for viewing images. Its image manipulation capabilities are sufficient to meet the needs of the average user. For instance, Batch Rename allows you to change the cryptic default filenames digital cameras use to something more meaningful. It can also convert them to a format that requires less disk space. XnView also lets you crop, rotate, and resize photos; set brightness, contrast, hue, and saturation; apply various filters, such as blurring; enhance details; sharpen edges and focus; add embossing; add different types of noise, such as Gaussian, Laplacian, and Poisson; and add effects, including drop shadows, 3D borders, swirls, and mosaics.
You can use XnView to acquire images from a scanner, modify the number of colors in an image, edit metadata and image properties, and edit International Press Telecommunications Council (IPTC) data. Webmasters will appreciate that XnView creates Web pages with links to the images or thumbnails in the directory. You can also view a plain-text listing of the files in the directory.
XnView is not a full-fledged image editing program, but it's good for viewing a site or directory's images all at once. It's also useful when making screenshots, because it allows you to crop only the necessary part of a screen, save it in one of many formats, add text, and process the image a little. The average screenshot of a dialog box tends to feature gray as the dominant color, and many other programs tend to "enhance" the gray areas with subtle reddish stains. Other programs also sometimes save a file as a JPEG or GIF with so much loss of image quality that the file is beyond recognition. I've never experienced any of these problems with XnView.
My only complaint is that XnView doesn't let you draw arrows or circles on an image. I currently use the GIMP for arrows, circles, and other enhancements to screenshots. However, most users probably don't even need this feature -- people generally don't draw on their photos, right?