GIMP-GIMP is a popular open-source image editor originally developed for Unix/Linux. Often lauded as the "free Photoshop," it does have an interface and features similar to Photoshop, but with a steep learning curve to match. Because it's volunteer-developed beta software
Paint.NET- is a free image and photo manipulation software for Windows 2000, XP, Vista. Paint.NET started development at Washington State University with additional help from Microsoft, and continues to be updated and maintained by some of the alumni that originally worked on it. Paint.NET features layers, painting and drawing tools, special effects, unlimited undo history, and levels adjustments. Paint.NET is completely free, and the source code is also available for free.
Picasa-Google most downloaded software is picasa. it’s free and can be used on Linux, Mac or Windows. It has some basic editing functions, but it does not allow you to do pixel-level editing. it is primarily photo organizing software
PhotoScape-One of top picks by users. It is jam-packed with features, while remaining very easy to use. Photoscape provides several modules including a viewer, editor, batch processor, Raw converter, file renamer, print layout tool, screen capture tool, color picker, and more.
other top picks include Serif PhotoPlus, IrFanView etc