Sometimes it’s useful to be able to open the file you’re editing in Emacs in an external program. For instance - you might be editing some HTML file and you might want to see how is it looking in a browser. I use the following handy command to do so:

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(defun prelude-open-with ()
  "Simple function that allows us to open the underlying
file of a buffer in an external program."
  (interactive)
  (when buffer-file-name
    (shell-command (concat
                    (if (eq system-type 'darwin)
                        "open"
                      (read-shell-command "Open current file with: "))
                    " "
                    buffer-file-name))))

On OSX it would use the open program to decide which program to open the file with, otherwise you’d be prompted to enter the name of the program in the minibuffer.

I find it convenient to bind the command to C-c o:

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(global-set-key (kbd "C-c o") 'prelude-open-with)

This command is naturally part of Emacs Prelude.