Growl is a notification system that allows any application to send it a notification, and then it displays it. You can (for example) be notified that you have new email, or that a download completed.

WHAT'S NEW
Version 1.4:
General:

Applications Tab Redesign! This is a major overhaul, and should be much easier to use.
Moved Rollup controls to separate tab.
Major improvements to networking.
Preference Pane overhaul:
New slider switch for on/off switches
Added keyboard support to position picker.

General Tab Relayout.
Networking tab
Add support for subscription
History Tab, contextual menu for history table to jump to the app/notification settings
Rollup tab is the new home of controls related to the Rollup window.
Adding a new display should not require a restart of Growl now.

Networking:
Support for Notification-Callback-Target, a URL field in GNTP. Upon click, Growl lets OSX figure out how to open the URL (such as the default browser for an http:// url)
Subscription support, subscribe to all applications on a remote machine.
General improvements to networking authentication, removed requirement for, and use of password headers if the password field is blank
Fix networking crash related to address data resolution.
Bind to local socket only if not listening for incoming notifications.
Address data caching, and observation of network changes to clear caches

Applescript:
URL click feedback support, same as Notification-Callback-Target in GNTP
Bug fixes:
Resolved a crash with webkit plugin bridge on creation.
Resolved a crash with forwarding.
Resolved an issue where the Speech display waits for previous message to complete causing a CPU drain
Resolved a crash with CFErrorGetDomain for WebKit Display Styles.
Fixed duplicate listings of display plugins on reinstall.
Notification-Callback-Target is now respected, and used, even without the method header.
Worked around problems with corrupt history databases.
Fixed a sorting problem when deleting a single entry in the history list.