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Here's a little pattern I use quite often in web applications. Like a lot of patterns, it's not tied to any particular environment, or even to any specific application. There's a lot of these kinds of patterns out there that are specific to web applications (in addition to the standard patterns), but they don't seem to be well collected, defined, or named...

Frequently you will need to present out-of-context information to the user. For instance, in a webmail application when sending an email. A page saying "email successfully sent" isn't very interesting. Simply redirecting the user to the email index isn't very friendly either: it's important to give some feedback when a user performs a state-altering action. You want to predict what the user is likely to be most interested in, and direct them there, but you also want to put some notification about what just happened.

In whatever framework I am using (Webware, Zope, PHP), I'll usually add something like this to the template header (Webware here):

class SitePage(Page):
    ....
    def writeMessage(self):
        if self.session().value('messages'):
            self.write('<div class="alert">%s</div>' %
                       '<br>\n'.join(session.value('messages')))
            self.session.setValue('messages', [])
    def sendMessage(self, msg):
        self.session().setValue('messages',
            self.session().value('messages', []) + [msg])

<div tal:condition="request/SESSION/message | nothing"
     style="border: thin black solid; background-color: #ddddff">
  <tal:block repeat="msg request/SESSION/message">
    <span tal:replace="msg" /><br>
  </tal:block>
 <tal:block replace="python: request.SESSION.set('message', [])" />
</div>

If you haven't seen tal:block before, any tag prefixed with tal: gets removed from the final ZPT output, and any attributes of that tag are treated as TAL attributes. It saves a bit of typing, and anyone using ZPT can use all the help they can get to save typing.

People sometimes implement this in different ways. For instance, they simply invoke the template passing in a message variable. So in a webmail application, this might look like:

# send mail action:
invalid_message = validate_email(request) # return None if valid
if invalid_message:
    return sendmail_form(message=invalid_message)
sendmail(user.email, request.to, request.subject, request.body)
return mail_index(message='Email to %s sent successfully' %
                  request.to)

A solution to this is to redirect the user after sending their email. There's a few gotchas here relating to exactly what result code you use (there's a whole set of 3xx result codes), but that's another topic.

People sometimes start passing the messages in GET variables, e.g. redirect('/index?message=Email+sent+successfully'). This is terribly awkard, and it's not worth it to go down this path. Just use sessions, which are perfectly suited for this task. The GET variable is also flawed because that message is forever tied to that page, even after a reload, and even after the message is no longer timely.

Once you integrate this into your templates, you'll find yourself using these messages all the time. You can use it to add quick error messages, where before you might have been too lazy, or just have used some sort of generic "There was an error, hit the back button on your browser and correct it" page. You can notify the user of all sorts of little events which you might otherwise simply imply (e.g., "item deleted from cart", "edits saved", and so on). These updates give the user a clear idea of what they've done, what they need to do next (because they usually are in the middle of some longer set of tasks only they understand), and generally that their efforts are successful.