Gimp for Mac and Gimp for Windows is one of the most popular free open source image editors around and quite rightly - it's remarkably powerful and easy to use for a free image editor. However, those Mac users used to the the luxurious surrounds of Photoshop probably have a hard time a) using a program called "Gimp" to edit their work and b) using the rather amateurish interface and weird naming conventions that Gimp uses.
If you wish you could combine the two in someway then you should definitely checkout GIMPshop for Windows or GIMPshop for Mac. GIMPshop emulates the look and feel of Photoshop remarkably well and changes all those weird Gimp labels such as "convolve" and changes it into the more familiar Adobe Photoshop term of "blur/sharpen". In fact, the program renames and reorganized almost all of GIMP’s tools, options, windows, and menus to closely resemble Adobe Photoshop’s menu structure and naming conventions. In addition, many of the menu options and even whole menus have been recreated to faithfully reproduce a Photoshop-like experience.
Of course, GIMPshop is no replacement for Photoshop in terms of functionality but it does a pretty impressive job looks wise.