GIMP

GIMP The GIMP is a free multiplatform photo manipulation tool, which is similar to Adobe © Photoshop. GIMP is an acronym for GNU Image Manipulation Program. The GIMP is suitable for a variety of image manipulation tasks, including photo retouching, image composition, and image construction.

It has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.

GIMP is expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.

GIMP is available for download here.
Note: You will need to download "The GIMP for Windows" and the proper "GTK+ 2 Runtime Environment" for your version of Windows.

You may want to also get the Windows Gimp Deweirdifyer plugin, which makes it look more like Photoshop. For a Windows PC, you should download the "BackgroundWindow-2.1.zip" file, unzip it and copy the files to you GIMP plugins folder. You can download the file here.

Another useful plugin is PSPI, which allows you to use 3rd Party Photoshop plugin filters in GIMP. You can find it here. Make sure you download the one that says "Windows executable".