Mac OS X Tiger/System Preferences/Appearance
The Appearance pane of System Preferences lets you adjust the little bits and pieces of the Mac interface. While the changes can be seen as minor, they can make a big impact on your Mac's feel.
The most important controls are the two pop-up menus at the top.
The first is "Appearance", which controls the color of accents in Mac OS X. The default setting, "Blue", is a bit of a misnomer. Not every accent in Mac OS X is blue (for instance, the buttons in window title bars are red, yellow, and green). It would be much better to call this setting "Color". When this pop-up is set to "Graphite", the life is sucked out of the Mac interface. Buttons, scrollbars, the Apple menu, etc. all turn a shade of grey.
This is a very niche option. The colored accents in Mac OS X are very appealing, and almost all users prefer to leave them turned on. Graphite is mainly used among graphic designers, who find extra color distracting and unprofessional. When used together with a neutral Desktop Picture, Graphite gives your Mac a very serious air.
The second pop-up menu lets you choose a highlight color, used when selecting text, windows in Exposé, files on the desktop, etc.
Other options let you customize how scroll bars work, how many items appear in > Recent Items, and how fonts are smoothed. These options are pretty self-explanatory.