gistfile1.el

(define-key global-map (kbd "C-c w") 'MyTool-lookup-dictionary-osx)
(define-key global-map (kbd "C-c g") 'MyTool-search-google)
(define-key global-map (kbd "C-c G") 'MyTool-search-googlescholar)

(defun MyTool-search-google()
  "Search by google"
  (interactive)
  (let* ((str (string-word-or-region)))
    (browse-url
     (concat "http://google.com/search?q=\"" str "\""))))

(defun MyTool-search-googlescholar()
  "Search string by google scholar"
  (interactive)
  (let* ((str (string-word-or-region)))
    (browse-url
     (concat "http://scholar.google.com/scholar?q=\"" str "\""))))
  
(defun MyTool-lookup-dictionary-osx()
  "Look up the word by Dictionary.app of Mac OS X"
  (interactive)
  (let* ((str (url-hexify-string (string-word-or-region))))
    (browse-url (concat "dict://" str ))))

(defun string-word-or-region ()
  "If region is selected, this returns the string of the region. If not, this returns the string of the word on which the cursor is."
  (let ((editable (not buffer-read-only))
        (pt (save-excursion (mouse-set-point last-nonmenu-event)))
        beg end)
    (if (and mark-active
             (<= (region-beginning) pt) (<= pt (region-end)) )
        (setq beg (region-beginning)
              end (region-end))
      (save-excursion
        (goto-char pt)
        (backward-char 1)
        (setq end (progn (forward-word) (point)))
        (setq beg (progn (backward-word) (point)))))
    (buffer-substring-no-properties beg end)))

Original code: http://sakito.jp/mac/dictionary.html

Searching by Google scholar by using URL `http://scholar.google.com/scholar?q="...."' may be also useful.