Terminal Hacks:
Terminal Hacks are usually codes that you enter in the Terminal of your Mac to unlock hidden features that can allow you to tweak appearance-features of your Mac!
What you must do is copy-paste the given codes in your Terminal as shown.
I will list my favored and most useful Terminal hacks and then tell you were to find many more.
Note: To use the code, you must enter it (copy-paste) correctly and then also "Kill" the desired application to view the changes (you will be told if you need to).
Also, note that any of these commands can be turned back to the OS X's original settings by changing the YES to a NO.
e.g.:
defaults write com.apple.dashboard devmode YES
To "uninstall" this hack, simply type:
defaults write com.apple.dashboard devmode NO
Accessing Terminal and setting up:
To access the Terminal, go to:
Spotlight, enter = Terminal > Select "Terminal" > You're now on the main Terminal interface!
To access Spotlight, simply click on the spyglass at the top-right of your screen, Spotlight is just the name given to Mac OS X's powerful search-tool
Now that you can access the Terminal, consider placing the application icon at the bottom of your screen in your Dock for future use. Now to improve the look or your terminal, go to:
Terminal > Preferences > On startup, open: new windows with settings > Pro.
Now quit and relaunch Terminal for the nicer-looking version!
Here is the basic Terminal screen (Leopard is the name of my Mac, I named it thus 3 years before OS X 10.5 was released - I simply chose another name of the wild feline family, uncanny huh?):

Killall feature:
This isn't actually a code to Pimp your Mac, but you will use it in the future to "kill" your Dock/Finder/etc. What it does is basically quit, and then for programs that systematically must be open (Dock and Finder, etc.), it restarts them.
You must use a capital for the name of the Application, e.g.:

This will therefore relaunch (for example) the Dock once you've altered it.
2D Dock switching:
Before you could get a program to do it for you, the only way you could switch your Dock from 2D to 3D was by using this handy piece of code:
defaults write com.apple.dock no-glass -boolean YES
Followed by:

Dragging Widgets onto the Desktop:
This is truly one of the best. What it does, is it allows you to Drag any of your widges (custom downloads inclusive) onto your desktop to permanently place where-ever you like! This means that you can have a lovely widget on your Desktop at any time telling you the weather, upcoming movies or what song you're currently running in iTunes. No need to open up Dashboard ever time now!
defaults write com.apple.dashboard devmode YES
Setting a screen-saver as your Wallpaper:
This clever piece of code tells your Mac to fetch your current screen-saver as your wallpaper, perfect for showing off to friends or making an ultra-stylish Mac.
Note: You must keep Terminal open for this one to work! This means that when you've activated the code, simply Hide Terminal with Command + H to clean up your desktop of clutter. If you need to enter more code, open up a new Tab in Terminal with:
Shell > New Tab > Pro (or which-ever you prefer).
/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background
The great thing is that this works with any custom screen-saver you can get to work on your Mac. I found a Water Puddle effect, and used it on my nice background to create a lovely moving wallpaper.
Turn Hidden applications translucent in the Dock:
This one may seem superficial, but I never used the Hide feature until I found this lovely code. What it does it when you chose to Hide an application you're running, which can also be done by hitting Command + H, the application's icon in the Dock now turns translucent so you can instantly recognize which applications are hidden!
defaults write com.apple.Dock showhidden -bool YES
And then Kill the Dock.
Those are just a handful of the Terminal Hack codes you can find on the internet, but they are what I consider to be the most useful, and hopefully they'll get you started on using the powerful feature that is the Terminal!
Here is a site with what they call the "Top 50" of Terminal Hacks.
~ Sean