So I had an image (400x298 pixels) and I needed to make it square...
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Bumbled about for a few minutes with the Crop tool, then I remembered about the Liquid Rescale plugin for GIMP (which is now in the repositories). Whipped that out, told the thing to make the image 298x298 pixels, and it magically made everything fit perfectly without losing me any detail and without mangling the content! This stuff is great :)
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Edit: The plugin adds Liquid Rescale to the Layer menu.