All the way since back when I started using a Mac, the program that's been most recommended to new Mac users in my experience isn't Adium, Transmit, or Quicksilver (although those are certain all terrific apps/reasons to be a Mac owner) -- Cocktail is the one that I've seen on the Top 10 Mac apps most often. And there's no question that while Adium and those other apps might be flashier or more UI-driven, Cocktail is the nitro you toss in your car's fuel line to supercharge all of the stuff under the hood. From disk repairs and standard maintenance, to network optimization and cache and log file clearing, Cocktail is exactly the kind of program you buy a Mac for -- it gives you control over almost everything going on in your computer.
They've just updated to 4.1.4, with a whole slew of optimizations to show for it -- the application itself has shrunk in size by 50%, and there are lots of upgrades for faster and more reliable performance. Additionally, compatibility with FAT32 volumes have been addressed, and they've upgraded Automator actions, so they've given you even more ways to control everything you've got.
Great app, well worth the $14.95 for a single-user license. All the other signature Mac apps will let you do your daily tasks in style and with a little flair, but Cocktail is the one that will help you do them quickly and cleanly.