There are three good choices here, and the first of these is the classic IrfanView. Irfan is a first-class product, but one for which I have mixed feelings. It's an amazingly capable product and very fast at displaying images, something It consider essential for a viewer. Some additional features (its resizing algorithm, for instance) are excellent and even rank above a big fish like Photoshop. But although other users just love it, and the lack of wheel or quick zooming ability is a major handicap.
A product You feel more comfortable with, is XNView. Like Irfan, it's very versatile; it can read and display nearly 400 types of graphic files, and convert any of these to more than 50 formats. It displays images very quickly, and these may be viewed full screen, as slideshows or as thumbnails. It's quite capable at processing images, too; you can adjust brightness and color, apply filters or effects, crop and resize, and more.
These operations can also be carried out from a batch file, which makes XNView ideal for converting large digital images to smaller sizes for use on the web or in emails. It supports drag and drop, has many plug-ins, is available in 44 languages and has full cross-platform support, including Mac.
XNView has a lot of similarities to IrfanView; so many in fact, that it comes perilously close to plagiarism. Put positively, one may say that imitation is the sincerest form of flattery; however, the developers of IrfanView must feel more than a little miffed. XNView is free for non-commercial use, and supports all Windows versions plus many other platforms.
My third choice is FastStone Image Viewer. This is a speed demon with a zippiness in displaying images that's reminiscent of the old ACDSee before it suffered feature bloat. But this zippiness disappears when it deals with larger files (>20Mb, depending on the format) and both Irfan and XN perform much better in this field. It supports all major graphic formats and popular digital camera RAW formats as well. It also has good basic image editing facilities, great slide show capabilities and an interface which is a delight to work with. Much to like here.
FastStone has a "little brother" called FastStone MaxView which is essentially the same program, but it's limited to the viewer and a few features.
A long comparative review of FastStone and XN can be found in the comments section below.
IrfanView
Website: http://www.irfanview.com/
Download link: http://www.irfanview.com/
Author: Irfan Skiljan
Current version: 4.10
Version date: November 21, 2007
License: Freeware for private use
Download File size: 1.10Mb
Operating Systems Supported: 95/98/NT/2000/ME/XP/2003/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: Yes
Non-English languages supported: Multi language support
Other relevant information: Extensive plugins available
Layers are not supported
XNView
Website: http://www.xnview.com/
Download link: http://pagesperso-orange.fr/pierre.g/xnview/endownloadwin32.html
Author: Pierre-e Gougelet
Current version: 1.93.6
Version date: May 1, 2008
License: Freeware for private use
Download File size: Basic 2.11Mb, Standard 4.85Mb, Plugins 3.53Mb
Operating Systems Supported: Windows 95/98/NT/2000/ME/XP/Vista
Windows 3.x, Mac OS X, Linux x86/Linux ppc, FreeBSD x86, OpenBSD x86, NetBSD x86, Irix mips, Solaris sparc, Solaris x86, HP-UX, AIX,
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Additional Software Required: None
Non-English languages supported: French, German
Other relevant information: Multi-language with D/L of additional language pack
FastStone Image Viewer
Website: http://www.faststone.org/
Download link: http://www.faststone.org/FSViewerDownload.htm
Author: FastStone Soft
Current version: 3.5
Version date: January 15, 2008
License: Freeware for private use
Download File size: 4.24Mb, portable 4.18Mb
Operating Systems Supported: 98/2000/Me/XP/2003/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: Yes
Non-English languages supported: Multi language support only in version 3.2 and older: Chinese(Simp & Trad), Danish, Dutch, French, German, Hungarian, Italian, Norwegian, Polish, Portuguese, Russian and Spanish
Other relevant information: Supports dual-monitor configurations
Tutorial available: 3.7MB