Cocktail is both useful and fun for inveterate Mac tinkerers and would-be power-users (that is, power users who aren't hard-core enough to use the command line). This multipurpose utility can help you keep your computer happy and healthy, giving you access to scores of tweaks and optimizations pulled together from various utilities, preferences, third-party hacks, and command-line tools.
Cocktail can help you with routine maintenance and customization across your disks, system, files, network, and interface. Cocktail is organized around these five areas, with highly customizable options for everything from clearing caches and repairing permissions to changing the appearance and behavior of the Finder and Dock--and Cocktail's Pilot feature lets you schedule regular maintenance.
Perhaps the best part of Cocktail is its extensive help files, cross-linked to articles on Wikipedia, Apple's site, and other Mac resources. These fall short in some spots (such as with optimizing network settings), but they can provide a valuable primer on how your Mac works. There are other cheaper--and even free--options out there, but with Cocktail, you get OS-specific stability, helpful documentation, and a well-organized interface. If you're not using Snow Leopard, check out the earlier editions of Cocktail.