Tweaking OS X
Onyx (Leopard or Tiger) offers a set of tools similar to Cocktail's, with some minor differences. OnyX can help you with everything from checking S.M.A.R.T. status on your disks to configuring the Dock, Finder, Dashboard, and Expose. One of my favorite tweaks with OnyX is the ability to have an animated desktop background. You won't get the scheduled, routine maintenance that comes with Cocktail's Pilot feature, but you may be able to look past it, because, well, OnyX is free.
Even though Mac OS X is a great experience from its first launch, sometimes people like to get under the hood and make customizations of their own. Thankfully, there are programs like OnyX and Cocktail to keep the guesswork out of small tweaks and routine maintenance, making it possible for people like you and me to make tweaks of our own.