In an ideal world, you would never need to make your browser pretend to be a different browser. In reality, a number of websites check for specific browsers such as Mozilla or Internet Explorer, or even specific versions of those browsers. Other websites check for popular search engine crawlers such as the Googlebot in order to display content optimized for that search engine. You may want to change your user agent to work around such limitations, or you might want to change your user agent string just for fun.

The following code allows you to set your user agent (wicked/w3m-set-user-agent), reload the current page using a specified user agent (wicked/w3m-reload-this-page-with-user-agent), and define regular expression matches for URLs to control user agent strings (wicked/w3m-fake-user-agent-sites). To use this, add the following to your ~/.emacs:

 (defvar wicked/w3m-fake-user-agents ;; (1)
   `(("w3m" . ,(concat "Emacs-w3m/" emacs-w3m-version " " w3m-version))
     ("ie6" . "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)")
     ("ff3" . "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.1) Gecko/2008070206 Firefox/3.0.1")
     ("ff2" . "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.13) Gecko/20080208 Firefox/2.0.0.13")
     ("ie7" . "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727)")
     ("ie5.5" . "Mozilla/4.0 (compatible; MSIE 5.5; Windows 98)")
     ("iphone" . "Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_0 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5A347 Safari/525.20")
     ("safari" . "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_5_2; en-us) AppleWebKit/525.13 (KHTML, like Gecko) Version/3.1 Safari/525.13")
     ("google" . "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)"))
   "*Associative list of user agent names and strings.")

 (defvar wicked/w3m-fake-user-agent-sites ;; (2)
   '(("^https?://www\\.useragentstring\\.com" . "ff2"))
   "*Associative list of regular expressions matching URLs and the agent keyword or value.
 The first matching entry will be used.")

 (defun wicked/w3m-set-user-agent (agent)
   "Set the user agent to AGENT based on `wicked/w3m-fake-user-agents'.
 If AGENT is not defined in `wicked/w3m-fake-user-agents', it is used as the user agent.
 If AGENT is empty, the default w3m user agent will be used."
   (interactive
    (list
     (completing-read "User-agent [w3m]: "
                    (mapcar 'car wicked/w3m-fake-user-agents)
                    nil nil nil nil "w3m"))) ;; (3)
   (if agent
       (progn
        (setq w3m-user-agent
              (or
               (and (string= agent "") (assoc "w3m" wicked/w3m-fake-user-agents)) ;; (4)
               (cdr (assoc agent wicked/w3m-fake-user-agents)) ;; (5)
               agent)) ;; (6)
        (setq w3m-add-user-agent t))
     (setq w3m-add-user-agent nil)))

 (defun wicked/w3m-reload-this-page-with-user-agent (agent)
   "Browse this page using AGENT based on `wicked/w3m-fake-user-agents'.
 If AGENT is not defined in `wicked/w3m-fake-user-agents', it is used as the user agent.
 If AGENT is empty, the default w3m user agent will be used."
   (interactive (list (completing-read "User-agent [w3m]: "
                    (mapcar 'car wicked/w3m-fake-user-agents)
                    nil nil nil nil "w3m")))
   (let ((w3m-user-agent w3m-user-agent)
       (w3m-add-user-agent w3m-add-user-agent))
     (wicked/w3m-set-user-agent agent) ;; (7)
     (w3m-reload-this-page)))

 (defadvice w3m-header-arguments (around wicked activate) ;; (8)
   "Check `wicked/w3m-fake-user-agent-sites' for fake user agent definitions."
   (let ((w3m-user-agent w3m-user-agent)
         (w3m-add-user-agent w3m-add-user-agent)
         (sites wicked/w3m-fake-user-agent-sites))
     (while sites
       (if (string-match (caar sites) (ad-get-arg 1))
         (progn
           (wicked/w3m-set-user-agent (cdar sites))
           (setq sites nil))
       (setq sites (cdr sites))))
     ad-do-it))

wicked/w3m-fake-user-agents sets up a number of common user agents(1) using examples from http://www.useragentstring.com. If you frequently use other user agents, add them to this associative list. wicked/w3m-fake-user-agent-sites sets up some rules for URLs so that you can work around specific websites(2). The first matching rule will be used.

wicked/w3m-set-user-agent can be called from a w3m browser session to set the user agent for all new pages visited. By default, it uses the w3m user agent(3). It will also use the w3m user agent if the agent is blank(4). If the user agent is one of the frequently-used agents defined in wicked/w3m-fake-user-agents, then the corresponding user agent string will be used(5). If not, the string will be used as-is(6). If the agent is nil, the user agent string will be disabled.(7)

You can check a single page using a different user agent by using M-x wicked/w3m-reload-this-page-with-user-agent. It temporarily sets the user agent and then reloads the current page.(7)

The last segment of code modifies the behavior of w3m-header-arguments(8), matching wicked/w3m-fake-user-agents against the URL. This temporarily sets the user agent for matching sites.