Hi all. To save others time, I'm going to post some helpful links for customizing your GUIs. Please feel free to contribute links to other programs and techniques.
Boot Logo: BootXChanger. Change the default gray Apple logo to
something else.
Desktop Widgets/Geeklets: GeekTools. This is what's usually used to make nifty background-level widgets that show calendars, weather reports, moon phases, processes, etc. Each of these widgets are known as "geeklets." Google will reveal several sites with compilations of various geeklets, like
this one.
Dock: Several applications exist for customizing your dock and/or managing several themes at once. Here are a few:
Dock Library,
Docker,
LeopardDocks, and
Mirage.
Finder Windows: Themepark. Tool for customizing Mac OS's look and feel. Customize, export, import themes.
Instant Messaging: Adium. There's no better chat client for customizing. You can change nearly everything about it.
iTunes: Bowtie. Highly customizable iTunes controller.
Login Screen: Ravissant. Customize the logo, greeting message, etc. of login screen.