System information can be useful for most people. For example, you can check your RAM configuration before buying additional one, etc.
Speccy (currently beta) is small, portable and freeware utility from Piriform (makers of CCleaner). It will give detailed information about CPU, Motherboard, RAM, Graphics Cards, Hard Disks, Optical Drives and Audio support.
Additionally Speccy adds the temperatures of your different components, so you can easily see if there's a problem.