System Spec is a free utility that displays exhaustive information about your system, including details on memory usage, CPU speed, options and features your graphic card supports (or doesn't), and much more. It also includes access to a wide range of system functions: for example, you can bring up any Control Panel module, or eject a CD.
Unfortunately, despite claims of being Windows 7 "compatible," a number of functions do not work properly in that environment. Testing on a Windows Vista system confirmed that the functions do work in other environments. The developer has acknowledged the Windows 7 issues and is in the process of correcting them.
That aside, what does System Spec give you? A whole lot. Each of the many pages (CPU, memory, network, and more) is filled with detailed information on the appropriate topic. The CPU page, for example, tells you not only the speed, but the vendor, the revision code, and much more.
While this is mostly of minor interest to a casual user, this level of detail is valuable in a few key circumstances. Trying to deal with an obscure bug or oddity, especially with very new or very old software, often requires you to find specific system details. Many games, in particular, may require a patch or special setting to deal with certain hardware configurations. (For example, when I reviewed Cube 2: Sauerbraten, knowing driver details for my graphics card let me fix a crash bug.)
Also, since System Spec is designed to fit on and run from a USB key, anyone who does tech support can quickly get a "readout" of a machine, and IT departments can request that users generate and then e-mail such reports, which can help track hardware problems.(System Spec can export data to a CSV or to HTML, or just copy it to the clipboard so you can paste it into a form or document.
If you're using Windows 7, I'd recommend holding off on System Spec for now. If you're using an older version of Windows and have ever needed to know this sort of information, System Spec is a good bet. It's free and simple to use.