Some IrfanView features:
Many supported file formats (click here the list of formats)
Multi language support
Thumbnail/preview option
Slideshow (save slideshow as EXE/SCR or burn it to CD)
Show EXIF/IPTC/Comment text in Slideshow/Fullscreen etc.
Support for Adobe Photoshop Filters
Drag & drop support
Fast directory view (moving through directory)
Batch conversion (with image processing)
Email option
Multimedia player
Print option
Change color depth
Scan (batch scan) support
Cut/crop
IPTC editing
Effects (Sharpen, Blur, Adobe 8BF, Filter Factory, Filters Unlimited, etc.)
Capturing
Extract icons from EXE/DLL/ICLs
Lossless JPG rotation
Many hotkeys
Many command line options
Many PlugIns
Only one EXE-File, no DLLs, no Shareware messages like "I Agree" or "Evaluation expired"
No registry changes without user action/permission!
and many more