I'm with todd, GIMP does a pretty perfect job (at least for the money
). Its basically designed for the same taskes like PS, but of course hasn't the whole load of Features and tools. But to be hornest, as a photographer (and not a grapic designer) I can do everything I want.
If you are looking fpr a good RAW- converter and Tool to organise your photos (not only RAWs), I strongly recommend the young project
darktable. There are stabile versions for nearly all linux distributions and OS X, I think windows will come some day too...