GIMP is a free, open source alternative to Photoshop, supports most of the features and can be used on Linux, Windows, Unix and Mac too!
I have used Gimp to do basic touch up jobs, its pretty easy and powerful too. I definitely found it easier to get around than Photoshop.
Start using Gimp and if you need help to learn more features, download the tutorial (Under 1 MB/PDF).
Gimp can be downloaded from here.
For the die hard Photoshop fans who refuse to use anything else, you can still try GIMPShop. GIMPShop is a version of GIMP which looks and feels like Photoshop, so that Photoshop users would feel at home. What more could you ask for 🙂