GIMP stands for "GNU Image Manipulation Program" and is a free alternative to Photoshop.
It has a full suit of photo editing tools, so you will not feel like you are using a substandard product.
GIMP can be used for: photo editing, image authoring, painting, batch processing, animating, etc...
It works on Windows, Mac, and many Linux operating systems.
There is also a plugin registry that contains even more tools.