A glowing plastic cube that lights up when you have mail in your Gmail inbox, via a Boarduino MCU:
The following guide is deliberately fairly high-level, because the exact details will vary depending on your operating system and particular hardware setup. I did this with my Mac, but hopefully there’ll be enough information here for you make it work on your system, perhaps with a little Googling.
If you don’t happen to have a glowing cube lying around, you can modify this to work with almost any output device you could think of, from a simple LED, or a buzzer, to something far more clever like moving a servo (Gmail Notifier Robot, anyone?)
How to make a Physical Gmail Notifier – Link