Whenever we write about Mac OS X utilities for system maintenance, our readers always remind of us one freeware utility that does a lot of work at no cost -- OnyX from Titanium Software.
This multifunction application does verification of your Startup Disk and System file structure, performs a lot of miscellaneous tasks for system maintenance and cleanup, and lets you configure some hidden parameters for the Finder, Dock, Dashboard, Exposé, Safari, the Login window, and even some of Apple's applications.
The improvements in OnyX 2.0 include faster operation, and deleting the applications cache, Internet cache, and logs has been improved. Most of these improvements are due to the application now being complied with the latest version of Xcode. One new feature lets you rebuild Mail's envelope index in the automation panel.
While the version 2.0 download is only available for Leopard, you can also download earlier versions for Tiger, Panther, and Jaguar. Support is pretty good; it's done through Titanium's forums and there are always answers available for most of your questions.
If you don't use OnyX, what's your favorite free Mac system maintenance utility?