OpenWith minor mode by Markus Triska.

Open files with external applications. Works from everywhere (find-file, dired, etc.).

If you also use emacs for email, you may want to add this to your config:

(add-to-list  'mm-inhibit-file-name-handlers 'openwith-file-handler)

Otherwise openwith will wreak havoc with your outbound email attachment workflow.

If you have problems making it work on MS Windows, have a look at this tip.

RunAssoc is a similar (alternative) utility. It can be set up to work with Helm; details after the jump. Note, RunAssoc is worth considering because OpenWith is giving errors, even with further customizations (below).

I’ve made a couple of improvements to this, you can find them on bitbucket. It opens things as if you double clicked them in Explorer on Windows, and wraps commands in nohup on Unix so launched processes will not exit if Emacs exits.

In order to get that to run without an error, I had to reformulate the main handler. The style here is a bit ugly but it works. However, at the moment on recent (August 2016) GNU Emacs, this still triggers further errors with find-file.

(defun openwith-file-handler (operation &rest args)
  "Open file with external program, if an association is configured."
  (let ((continue t))
    (when (and openwith-mode (not (buffer-modified-p)) (zerop (buffer-size)))
      (let ((assocs openwith-associations)
	    (file (car args))
	;; do not use `dolist' here, since some packages (like cl)
	;; temporarily unbind it
	(while assocs
	  (setq oa (car assocs)
		assocs (cdr assocs))
	  (when (save-match-data (string-match (car oa) file))
	    (let ((params (mapcar (lambda (x) (if (eq x 'file) file x))
				  (nth 2 oa))))
	      (when (or (not openwith-confirm-invocation)
			(y-or-n-p (format "%s %s? " (cadr oa)
					  (mapconcat #'identity params " "))))
		(if (eq system-type 'windows-nt)
		    (openwith-open-windows file)
		  (openwith-open-unix (cadr oa) params))
		(kill-buffer nil)
		(when (featurep 'recentf)
		  (recentf-add-file file))
		;; inhibit further actions
		(progn (setq continue nil)
		       (message "Opened %s in external program"
				(file-name-nondirectory file)))))))))
    (when continue
      ;; when no association was found, relay the operation to other handlers
      (let ((inhibit-file-name-handlers
	     (cons 'openwith-file-handler
		   (and (eq inhibit-file-name-operation operation)
	    (inhibit-file-name-operation operation))
	(apply operation args)))))

OpenWith for interactive functions only

I found that openwith is also opening files when a file was opened non-interactively from package “dashboard”. I had to change the defun for openwith-file-handler to check (called-interactively-p):

(defun openwith-file-handler (operation &rest args)
  "Open file with external program, if an association is configured."
  (when (and openwith-mode (not (buffer-modified-p)) (zerop (buffer-size))
             (called-interactively-p)) [...]

See also CategoryExternalUtilities