If you're not already a pro at using the command line, OnyX is a solid choice for conducting routine maintenance and tweaking the interface on your Mac. This free utility shares much of the functionality of applications like Cocktail (which its help files suspiciously resemble), and it can help you with everything from checking SMART status on your disks to configuring the Dock, Finder, and Dashboard and Expose to your liking.
OnyX can even help you run Unix utilities, change your desktop to an animated background, and unlock hidden functions in apps like Safari and iTunes. As with all OS-tinkering applications of this kind, most features work most of the time, but your mileage may vary when it comes to effectiveness and stability--so buyer beware (especially given that OnyX doesn't cost anything). This version will only work with Snow Leopard, so check out earlier versions if you're using an older version of Mac OS X.