GIMP is a full featured Photoshop competitors. Even better, it is cross-platform (Mac, Windows, Linux) and FREE! Explore books and tutorials on GIMP. There is even a portable version that runs on a USB stick.
Already an experienced Photoshop user, and you don't want to re-learn another tool? No worries, enter GIMPshop:
If you’ve never used Photoshop before, you may not appreciate my GIMPshop hack. What I’ve done is renamed and reorganized GIMP’s tools, options, windows, and menus to closely resemble Adobe Photoshop’s menu structure and naming conventions. Many of the menu options and even whole menus were recreated to faithfully reproduce a Photoshop-like experience. After running my GIMPshop hack, you’ll find that Photoshop and the GIMP are strikingly similar.
Longtime Photoshop users should feel very comfortable using GIMPshop.
Read more about GIMPshop.