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Apart from Picasa's indisputable first position among image viewers, there's a very interesting competition in the Top Downloads list for this category on Softonic. Two of the most powerful candidates are FastStone Image Viewer and XnView, two popular image viewers that share many features but also have certain differences. Let's analyse both of them in depth.
When launching both programs, the first noticeable difference is their interface. Whilst XnView looks very much like Windows Explorer, with a tree-like structure on the left and the selected folder's contents on the right, FastStone Image Viewer uses a neatly designed interface that you can customize with skins. Both programs use thumbnails to display photos and ALT text windows to show detailed information about each picture.
Being image viewers, both FastStone and XnView focus their main function into displaying photos in a comfortable way. On the one side, XnView allows you to open more than one picture at the same time thanks to its tabbed interface. On the other side, FastStone offers a better performance when working in full screen mode, as it features a series of very handy menu bars to access editing tools, see EXIF data or navigate through the photo stream.
Both applications include a basic set of tools to edit your pictures (such as crop, resize, rotate, flip, etc.) and an advanced toolkit for further editing, which is more complete in XnView. 
They also share extra features like the ability to set any photo as wallpaper, a screen capturing tool and a red-eye removal tool, just to mention a few. However, XnView beats FastStone in the number of supported formats –more than 400, against the roughly 20 opened by its competitor.
So, when it comes to choose between FastStone Image Viewer or XnView, think about what your main needs are (do you fancy a colourful interface? do you usually review your photos in full screen? do you work with a lot of different formats? do you need many editing tools?) and just act accordingly. We're plumping for XnView.
interesting, thank you mate!
- If XnView had interface as F.S.I.V. (XP Steel for exaple) or if FastStone had a Preview/Thumbs window position option - as in XnView - the world would be a better place.
Worth mentioning is that xnview is an extremely fast cropper if you need custom non-standard crops for a lot of images. Left mouse button is cropper tool by default and Shift-X crops from the image viewer directly. If you change the appropriate line in the .ini file to "ShowSaveDlg=130" it'll lose the overwrite confirmation and make the process very, very straightforward and easy.
I use it for language learning. I like to play games and read scanned material in the target language and was looking for something that lets me just crop a piece of text from an image I've print-screened to a file using Cropper and save it without hassle. JPEGCrops came close but doesn't let you freely draw a marquee (you have to drag one around that's already there).
I don't like it for image viewing, though. I keept FastStone installed as well and use FastStone instead. Before that, I was all about ACDSee 3.1. FastStone is probably the closest in a modern program, and adds some nice filtering for zoomed images.
Woops. Couldn't actually get rid of the overwrite dialog until I used "SaveAdvDlg=1" in the [Start] section of the xnview.ini without changing "ShowSaveDlg" and it had to be 1.91 from oldapps.com. "SaveAdvDlg=1" gives you the option to never show the overwrite dialogue again when it appears, and 1.91 saves your choice; 1.98 does not.
...And you need to put "ShowSaveDlg=129" in the .ini for some completely inexplicable reason in addition to downgrading to 1.91.2 and adding the "SaveAdvDig=1" line. Endears me to the software less, but will be useful to anyone who requires this streamlined functionality (FastStone won't get you there no matter what you do.)