ImageWell is nice and free. It allows for simple photo editing. I use shutterbug from the same company and like it very much so I would guess ImageWell is nice as well.
Just open the file in preview and use the Tools menu: Selection tool to select a region and Crop (apple-K) to crop. Then save in the format of your choice.
Also LiveQuartz (http://www.livequartz.com/) is donationware if you decide you need to do more than you described.
One of the most overlooked apps for Mac OS X is ToyViewer. Thing is, it has been around longer than just about anything else on Mac OS X (ToyViewer was running on Mac OS X before it was called Mac OS X).
I've written about the Mac OS X version here and the Rhapsody version here. There are also NEXTSTEP and OPENSTEP versions available.
And the best part is that it is free!
Here are a couple examples of things ToyViewer can do beyond cropping...