Since Antarctica is very graphics-oriented company, I'm often doing graphical twiddling or fixup of one kind or another when the job isn't serious enough to call the real pros in. Back when I was on Windows, I mostly used Paint Shop Pro, which does a whole bunch of useful things in a straightforward way at a fair price. There doesn't seem to be anything equivalent on the Mac.
Last night, I wanted to do some transparency and colour munging on the masterpiece you see above, and I had to go home and use the Windows box there. Blech.
On OS X there's Graphic Converter, which comes with the Mac and is OK for brightening up digiphotos of your cute kittens, but isn't in PSP's league. Then, if I'm willing to sign up for a bunch of training and emit a whole lot of money, I can join the big leagues and get Photoshop and Illustrator. But that seems like overkill.
There's definitely an unfilled niche here in the software ecosystem. If someone knows what the tool is that I'm missing, drop me a line (my address is real easy to find) and I'll give it some free publicity.