GIMP - The best editor
Windows, Linux, or MAC
If it can't be done with GIMP it can't be done.
FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor. It has a nice array of features that include image viewing, management, comparison, red-eye removal, emailing, resizing, cropping and color adjustments. Its innovative but intuitive full-screen mode provides quick access to EXIF information, thumbnail browser and major functionalities via hidden toolbars that pop up when your mouse touches the four edges of the screen.
Other features include a high quality magnifier and a musical slideshow with 150+ transitional effects, as well as lossless JPEG transitions, drop shadow effects, image annotation, scanner support, histogram and much more. It supports all major graphic formats (BMP, JPEG, JPEG 2000, animated GIF, PNG, PCX, TIFF, WMF, ICO and TGA) and popular digital camera RAW formats (CRW, CR2, NEF, PEF, RAF, MRW, ORF, SRF and DNG).
Paint.NET is an on-going project at Washington State University, with features that match commercial applications, including support for layers.
ImageForge is a freeware image editor gives you a great set of tools for painting and editing images and photos.
VCW VicMan's Photo Editor
VCW VicMan's Photo Editor has a host of features similar to Adobe Photoshop.
Picasa is a powerful and free software that helps you instantly find, edit and share all the pictures on your PC.
I use it, I like it, mostly for quick and simple jobs
IrfanView is a very fast, small, compact and innovative FREEWARE (for non-commercial use) graphic viewer for Windows 9x/ME/NT/2000/XP/2003/Vista. It is trying to be simple for beginners and powerful for professionals.