Bell Aqua is a simple shell script that uses Speech Dispatcher to announce the hour on the hour every hour. It is now also somewhat resilient against putting your computer to sleep. (Previously, if there were 20 minutes until the next hour, and you slept, and then woke up with 10 minutes to go, it would continue counting down from 20 minutes, meaning it would announce the hour 10 minutes late.)

This requires Speech Dispatcher

Source code and instructions: