iAlert for Mac

From Angeman:

iAlert is a preference pane which allows you to display an alert with a transparent floating window whenever a particular event occurs and lets you interact with the application that launched it.

The built-in plugins can show alerts when :

• web sites are updated
• a download is completed in Safari or Firefox
• new articles are fetched in Safari RSS
• you receive new messages in Mail, Entourage or Thunderbird
• you receive new messages or your friends get connected on iChat
• a new song is played in iTunes

You can also set up alarms and timers.

The alerts are fully customizable and you can control their size, their position on the screen, their opacity, their color and their background picture.

iAlert supports plugins : you can use AppleScript, make a shell script or use the iAlertBridge framework in your cocoa application to add your own alerts (See iAlertUserGuide for documentation).