GIMP for Windows
GIMP is the most popular open-source image editor for Unix/Linux. Windows users can use GIMP with installing GTK additionaly.
XnView is one of the best freeware-photoeditor. It is free for private non-commercial non-profit, educational use. There are also many modules & functions like: Import, Export many file formats, Animated GIF, Animated ICO support, metadata support, EXIF auto rotation, Resize, rotate, crop support, Adjust brightness, contrast, Apply filters like blur / average / emboss, Apply effects like lens/wave, Fullscreen, Slide show with effects, Batch convert/rename, Create WEB Albums, Screen capture, TWAIN & WIA support (Windows only), Print support (Windows only), Drag & Drop support (Windows only)
One of the best free photo-editors for Windows. It is not just an editor and viewer, there are also many plugins like layout designer, screen-capture, color picker, batch processor, converter, renamer, etc...
Free and old version of PhotoPlus. It is similar to very old Photoshop/Paintshop versions. Working on all Windows versions.
Pixia is a popular & free japanese photo-manupulation freeware. It is available in many other languages including english.
One of good free picture-editor with many functions.
Easy to use mini graphic-editor with a couple of effects.
A free image-editor from Washington State University and Microsoft for Windows NT series including XP.
You can download old versions of current Ultimate Paint. It is more than MS-Paint.
ImageForge is a small image-editor.
Picasa is more organizer & viewer than editor. But there are also a couple of functions for fast editing.
Irfanview is more an image-viewer than a photo-editor. There are also some functions like resize, rename, convert, optimize etc for fast editing. There are also some graphic-effects like emboss included.
I find this list very helpful but none of the compares to Adobe Photoshop CS4 (if you have it).