"For our money, though, the most powerful commands are in the 'Parameters' section, where you can tweak OS X, the Finder and other features to work smarter and harder. You can turn off flashier features, change your browser behaviour and change log-in behaviours.
OnyX isn't completely foolproof. You're well advised to back up important data before you start using it to optimise your system. Still, it's much easier to use than the command prompt – and will cost you exactly nothing."