GIMP: Free Image Editing Software
If you've heard of any of these open source programs, GNU Image Manipulation Program (GIMP) is probably the one. Virtually duplicating Photoshop's feature set, GIMP is a great choice for free image editing software for Macs.
Some features of GIMP include a customizable interface and easy image corrections that are built in, such as correcting for perspective distortions (like when you take a photo of a tall building) and barrel distortions (these make straight lines look curved).
GIMP has a complete set of retouching tools, including a clone stamp and healing brush. As for external bits like graphics pads special mice, GIMP supports those too, so you don't have to give up pressure-sensitive retouching tools.
You can save in a full range of file formats, from the standard print formats of PSD and TIFF to web formats like JPG, GIF, and PNG. On top of that, you can save in the crappy Windows format BMP as well as a plethora of minor formats used for icons and the like, such as TGA, XWD, XPM, and PIX.
I know several people who swear by GIMP. Granted, they're Linux people, but we can forgive them their little peccadilloes. It's an impressive, free program that will do the trick for your raster image manipulation needs.