In iOS, whenever you view a document attached to an e-mail, you can click the Share button to send the document to a list of compatible applications, not just the preferred one. The same option is available in Mountain Lion whenever you use Quick Look to view a document.
When you display a document using Quick Look, you can now right-click on the "Open in xxxx" button, and see a list of compatible applications. This makes it easy to open, for example, a spreadsheet or graphic in your application of choice instead of the default application.
Mac OS X Hints
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