There would have been scenarios where we were too much involved in our work and missed other appointments. And to avoid such scenarios we can use the feature in Mac OS X Lion to tell us the current time in our preferred voice.
Click the Apple icon and select Open Date & Time Preferences.
You can also access Date & Time Preferences by clicking Apple icon -> System Preferences -> Date & Time.
In the Date & Time Preferences navigate Clock tab and mark the check box with label as Announce the time.
You can choose the period after the time needs to be announced. The available options include On the hour, On the half hour and On the quarter hour. If you want to voice to be other than the System Voice then you can click the Customize Voice button and select your preferred voice.