GIMP is probably the best free one that is close to Photoshop. (There are other good ones, but I don't think they are as powerful.) Its pre-installed filter selection is mediocre (although this is remedied easily enough by a site that has quite a few extensions/scripts/filters). GIMP does lack some of the features that Photoshop has, but the next version due out in a few months will add layer groups, a single window interface, and many other things. GIMP's current "multiple window interface" is a bit annoying if you don't have multiple monitors to spread things out over, but that should be a moot point after the next release.