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.

Mac: Notification Center on Mountain Lion is great for keeping track of all your notifications, but we're still waiting for our favourite Mac notification app Growl to update with support for Mountain Lion. While we're waiting, Mountain Growl is a free app that pushes Growl data to Notification Center.

One of the challenges with using the Windows version of popular notification system Growl is that it doesn't always play nicely with all applications. Skowler aims to fix one such challenge: using Growl to provide notifications from Skype.

Windows only: Open source notification system Growl for Windows has finally hit the 2.0 milestone, and with it comes a ton of new features that let you further customise the way you receive your pop-in corner notices.

Mac OS X only: Universal notification application Growl has released a significant update, particularly for folks who've upgraded from Leopard to Snow Leopard.

Linux: Ubuntu 9.04 introduced a set of upper-corner notifications similar to Growl, but they lack for customisation in both appearance and usability. Mumbles aims to bring a true Growl-like experience to Linux desktops.

Windows only: The latest update for Growl for Windows adds a slew of new features—including support for Thunderbird, Snarl, system monitoring, and even a Gmail notifier.

Windows only: System notification tool Growl for Windows is a slick, customisable, universal alert system for your applications—so you don't have to be jealous of your Growl-using Mac friends anymore.

Windows only: Free application Snarl is a universal notification system for Windows inspired by the popular Mac OS X notifier Growl. We mentioned Snarl once before, but the young project has improved a lot since then, with extensions supporting Firefox, Pidgin, Outlook, Sunbird, and more. Out of the box Snarl has built-in support for battery warnings, system events, and timer and clock support (like hourly reminders)—along with several attractive themes. It needs support for a wider range of apps if it's ever going to do everything Growl does, but it's got a good start. Snarl is free and open source, Windows only. Thanks Eric! Snarl