Alert is a Growl-workalike for Emacs which uses a common notification interface and multiple, selectable "styles", whose use is fully customizable by the user.

For module writers

Just use alert instead of message as follows:

(require 'alert)

;; This is the most basic form usage
(alert "This is an alert")

;; You can adjust the severity for more important messages
(alert "This is an alert" :severity 'high)

;; Or decrease it for purely informative ones
(alert "This is an alert" :severity 'trivial)

;; Alerts can have optional titles.  Otherwise, the title is the
;; buffer-name of the (current-buffer) where the alert originated.
(alert "This is an alert" :title "My Alert")

;; Further, alerts can have categories.  This allows users to
;; selectively filter on them.
(alert "This is an alert" :title "My Alert" :category 'debug)

For users

For the user, there are several variables to control when and how alerts are presented. By default, they appear in the minibuffer much the same as a normal Emacs message. But there are many more possibilities:

alert-fade-time
Normally alerts disappear after this many seconds, if the style supports it. The default is 5 seconds.
alert-default-style
Pick the style to use if no other config rule matches. The default is message, but growl works well too.
alert-reveal-idle-time
If a config rule choose to match on idle, this is how many seconds idle the user has to be. Defaults to 5 so that users don't miss any alerts, but 120 is also good.
alert-persist-idle-time
After this many idle seconds, alerts will become sticky, and not fade away more. The default is 15 minutes.
alert-log-messages
By default, all alerts are logged to Alerts (and to Messages, if the message style is being used). Set to nil to disable.
alert-hide-all-notifications
Want alerts off entirely? They still get logged, however, unless you've turned that off too.
alert-user-configuration
This variable lets you control exactly how and when a particular alert, a class of alerts, or all alerts, get reported -- or if at all. Use this to make some alerts use Growl, while others are completely silent.

Programmatically adding rules

Users can also programmatically add configuration rules, in addition to customizing alert-user-configuration. Here is one that the author currently uses with ERC, so that the fringe gets colored whenever people chat on BitlBee:

(alert-add-rule :status   '(buried visible idle)
                :severity '(moderate high urgent)
                :mode     'erc-mode
                :predicate
                #'(lambda (info)
                    (string-match (concat "\\`[^&].*@BitlBee\\'")
                                  (erc-format-target-and/or-network)))
                :persistent
                #'(lambda (info)
                    ;; If the buffer is buried, or the user has been
                    ;; idle for `alert-reveal-idle-time' seconds,
                    ;; make this alert persistent.  Normally, alerts
                    ;; become persistent after
                    ;; `alert-persist-idle-time' seconds.
                    (memq (plist-get info :status) '(buried idle)))
                :style 'fringe
                :continue t)

Builtin alert styles

There are several builtin styles, and it is trivial to create new ones. The builtins are:

Name	Summary
fringe	Changes the current frame's fringe background color
mode-line	Changes the current frame's mode-line background color
gntp	Uses gntp, it requires gntp.el
growl	Uses Growl on OS X, if growlnotify is on the PATH
ignore	Ignores the alert entirely
libnotify	Uses libnotify if notify-send is on the PATH
log	Logs the alert text to Alerts, with a timestamp
message	Uses the Emacs `message` facility
notifications	Uses notifications library via D-Bus
notifier	Uses terminal-notifier on OS X, if it is on the PATH
osx-notifier	Native OSX notification using AppleScript
toaster	Use the toast notification system

Defining new styles

To create a new style, you need to at least write a notifier, which is a function that receives the details of the alert. These details are given in a plist which uses various keyword to identify the parts of the alert. Here is a prototypical style definition:

(alert-define-style 'style-name :title "My Style's title"
                    :notifier
                    (lambda (info)
                      ;; The message text is :message
                      (plist-get info :message)
                      ;; The :title of the alert
                      (plist-get info :title)
                      ;; The :category of the alert
                      (plist-get info :category)
                      ;; The major-mode this alert relates to
                      (plist-get info :mode)
                      ;; The buffer the alert relates to
                      (plist-get info :buffer)
                      ;; Severity of the alert.  It is one of:
                      ;;   `urgent'
                      ;;   `high'
                      ;;   `moderate'
                      ;;   `normal'
                      ;;   `low'
                      ;;   `trivial'
                      (plist-get info :severity)
                      ;; Whether this alert should persist, or fade away
                      (plist-get info :persistent)
                      ;; Data which was passed to `alert'.  Can be
                      ;; anything.
                      (plist-get info :data))

                    ;; Removers are optional.  Their job is to remove
                    ;; the visual or auditory effect of the alert.
                    :remover
                    (lambda (info)
                      ;; It is the same property list that was passed to
                      ;; the notifier function.
                      ))

naclander/alert

For module writers

For users

Programmatically adding rules

Builtin alert styles

Defining new styles