There were two things I started missing when I switched from Windows to OSX: one is the program IrfanView for folder based image viewing and basic manipulation (resizing, cropping, rotation). Those functions are handled pretty well though with a combination of Finder's builtin viewer (hit space when a file is selected, then use arrow keys to navigate through the folder itself) and OSX Preview.
The other was an editor more advanced pixel work, without running smack into the cost and/or usability hurdles of Photoshop or GIMP. On Windows I enjoyed Paint.NET (after a decade of getting by with an ancient copy of Paint Shop Pro). It was tougher on OSX -- Pixelmator was somewhat ok, but did a lot of things oddly, and fell into the mistake of assuming its native format was preferable for everything. (Sometimes you just want to open a PNG, edit some pixels, and save it again, but that wasn't trivial in Pixelmator.)
Now, however, I think I might have found my new favorite: "Acorn". I didn't experiment with it before, but it (temporarily?) knocked down its price from $50 to $15, and it seems well worth it.
So: "Acorn" for OSX. Recommended, and well-priced. (For the time being at least.)