Here is the most popular clock sidebar gadget from the Windows 7 Personalization Gallery. It has been downloaded over 2.4 million times in the last two years since it was posted on 01 Oct 2008.
It is very functional and is much more than just a clock display as indicated in the gadgets page at the Windows Live Gallery:
Date & Time gives you a date and time display, calendar and events system, and alarm clocks! Multiple themes, customization options, and more!
• Date and time display
• Ability to add multiple clocks
• Analog clock options
• Calendar and events system
• Alarm clocks with lots of options
• Easy to use, manage, and customize