The GIMP will work if you don't have the money as it has many of the capabilities that Photoshop has. However, it is lacking in some areas such as it does not have the ability to work on 16-bit images. Also, maybe it was just me, but even though it has color management capabilities, it doesn't seem to embed icc profiles in the files which can be a real pain.
If these things don't bother you, then the GIMP is a very capable photo editing program. I used it for a quite a while before I could afford Photoshop.