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Depending on what Mac you have, it may have come with Graphic Converter That should have some kind of crop tool...
 
Preview does what you want

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...

toyviewer01.jpg



toyviewer04.jpg

And some of the included tool options...

toyviewer02.jpg
 
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