I say: A system tweaker allowing you to tweak common and hidden system settings. Very powerful - dangerous if misused, but as it comes with a comprehensive help system and an extensive and useful tips reference this rates as a MUST HAVE.
NOTE: X-setup went commercial after version 6.6, and although Xteq promised to keep the freeware version available, it's impossible to find on their website and appears to have been comprehensively removed from most of the major archives. I managed to find a couple of sources for my Alternative links, but once these are removed I'll be dropping X-setup from my recommended list
They say: "X-Setup Pro 8.1 is now available and it includes several enhancements as well as bug-fixes.... bla bla bla..."