One of the supposed benefits of using a Mac is "it just works," and I've generally found this to be true (as a relatively recent convert from years of problem-laden PCs). However, no matter how solid a computer's foundation, once it's got a few years worth of added applications, hardware changes, and operating system upgrades, there's bound to be some virtual gunk that needs cleaning. My Macbook Pro is four years old and I'm asking it to edit 1080p video; to do this, it needs to be in midseason form, and the gym I've been sending it to is Titanium Software's Onyx. Onyx allows you to "verify the Startup Disk and the structure of its System files, to run misc tasks of system maintenance, to configure the hidden parameters of the Finder, Dock and of some of Apple’s own applications, to delete caches, to remove a certain number of files and folders that may become cumbersome and more."
Know of an alternative that's better than Onyx? Let me know!