CSS Tweak!, as you might imagine, can optimize and clean up your CSS code with the drag and drop of a file. The widget will report how much smaller your file is going to get, and its description states that your new file is "saved in the same location as the original", so I'm pretty sure this means it doesn't overwrite the original, but proceed (and backup!) with caution.
CSS Tweak! appears to be free and is available from Andy Peatling at his site, CssDev.
Widget Watch: CSS Tweak!
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