OS X: We love Growl for its highly customisable instant notifications of what's happening in other apps. On sale now for just $0.99, the app is 75 per cent off its normal price.
As we mentioned the last time Growl went on sale (back in February), Growl supports Apple's Notification Center, allows you to receive notifications in multiple ways and lets you personalise the notifications' appearance.
Combined with many available plugins and similar apps, you can use Growl as part of your all-encompassing notification and alerts system.
Pick it up now at the Mac App Store. There's no word on when the price will go back.