There are many hidden system settings on your Mac.  Generally, they can be accessed through some Googling and Terminal commands.  However, Secrets has created a database of these various tweaks.

Moreover, for Leopard users, there is even a PrefPane available that installs in System Preferences.  Turning on/off any system tweak is just a click away, no Terminal needed.

Some of my favorite tweaks (in Leopard) are the 2-D dock, “suck” minimize effect, and translucent dock icons for hidden applications.

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