Picasa is simple yet powerful tool for those uninterested in the technical aspects of image manipulation. Includes basic color correction, cropping, and a variety of special effects. Not a tool for deep and detailed editing, but great for crop and correct.
GIMP is the open-source competitor to Adobe Photoshop.GIMP has excellent Color correction, channel mixing, advanced cloning, paths, and layered compositions. A bit difficult to transfer over to for those used to Photoshop, but is an excellent tool in its own right.
Photoshop is the commercial standard. Tools across photo editing software were all pioneered in here: layers, slices, and image correcting macros and filters.There are entire companies devoted to creating plugins for it. Photo by HVarga.
Beginning as a computer science project Paint.net has grown into a sophisticatedapplication in its own right. The interface is easy to pick up, and an unlimited undo function makes correcting your learning-curve mishaps a snap.
Lightroom wasn't designed to be a detailed pixel-by-pixel editing tool but focuses on being a digital darkroom allowing for corrections necessary to their workflows. Lightroom excels at batch work and advanced color balance corrections. Photo by M. Keefe.