Windows 9x/ME/NT/2000/XP/2003
IrfanView (pronounced earfanview) is a nice little (>10MB) application for graphic viewing, movie & audio playback, etc. It also has some image manipulation capabilities. Honestly, it would take too much space to list its features here. Best to check out the website.
A Powerful Piece Of Software
This program has become my graphic viewer of choice. It's fast, covers an extensive amount of file formats, and has a lot of neat features. Plus, it is not at all system heavy. The only requirement is that the computer is a Windows system.
While I mainly use it for viewing my graphics, it has a few other nice features I thought I should mention here. There are many plugins available for the program, all downloadable from the site, including features to playback Quicktime and Real file formats. This was a big sell for me, because I really don't like programs that bug you to purchase it, not to mention the system hogs that they can be.
However, I should note, that these plugins are the main reason that this application didn't get a five. They're very finicky, and will occasionally not work. I've ran into problems with Quicktime Livefeeds, and Real streams. Nonetheless, it tends to playback a captured stream/feed flawlessly.
Check out the site for a lot more information on its capabilities.