Hey, I admit I only knew this today! And probably would have never known it throughout my life. I've been using DOS long before DOS version 6.0. I've also been using "Command" to reach a DOS console in Windows 95 and before. I've used "cmd" since Windows 2000 and Windows XP, yet I never knew that the tab key had a special interesting function in the Windows console!
Do the following in Windows XP: Click on Start; click on Run... then type cmd to get to the command line prompt (DOS-like). Now type the first few letters of any folder and press the tab key. If that folder exists at the current place, Windows will automatically complete the name of the folder for you! I know this might be considered as a trivial thing, yet it is so handy, specially given the presence of long folder names at some times.
The more interesting thing about this issue and what really made me write about it is the fact that no matter how long you've been using tools and programs, there could be small things hidden from you that you never discover but by chance and can take really long for you to know.