GIMP (GNU Image Manipulation Program) is an open source image editor that is comparable to Photoshop. By no means is it a drop in replacement. But a lot of the core functionality is there, including layers, filters, plugins, paths and channels. There is a bit of a learning curve, especially because the majority of tutorials on image editing are based on Photoshop. But with a lilttle legwork you can find the comparable features and create the same effects in GIMP without the price tag of Photoshop.