Growl alerts, the attractive translucent notifications, are one of my favorite features of OS X providing a simple outlet for all kinds of warnings and notifications.
However, they don't work for when connecting hardware to your Mac which is where Hardware Growler comes in. Hardware Growler simply adds Growl alerts for when you connect any kind of device to your Mac whether it is a hard drive, keyboard, mouse etc. Hardware Growler supports FireWire, USB, Bluetooth, FileSystem Volumes and Network Interfaces.
Simple double click on the installer and it automatically installs Growl alerts for anything you connect or disconnect to your Mac. It works instantly with no further configuration required. It's nice to know that whenever I connect or disconnect a device, I now receive a notification with exact name of the device.
Hardware Growler isn't exactly an essential add-on for Growl but it's useful for those that constantly connect and disconnect devices.