Bartender4 is an action bar replacement/enhancing addon. It allows you to completely customize the look, size, location, amount and even properties of your action bars, often saving a lot of space and optimizing efficiency - or just making your UI more visually appealing. The addon also has an easy and quick to use keybinding feature, which allows for easy and fast setup, and which also has custom profiles. This mod can single-handedly change your whole gaming experience and allow you to easily customize your UI. I would personally say that Bartender(4) is the best action bar replacement/enhancement addon, but there are also other very decent ones, such as Dominos.
After "unlocking" your bars, all the ones that you have enabled (you can do this from the interface shown later in this post) turn green, as highlighted in the screenshot above. After placing your bars where you want them, you can lock their positions by pressing the button "Lock", resulting in something like the screenshot below.
Hint: The bars that have nothing in their buttons may seem to disappear, if the "Button Grid" option is unchecked in configurations.
Using the mod's advanced interface, you can easily tweak any and all bars to suit your specific needs. Certain variables that are at your disposal include padding, the number of buttons/rows, the visibility of the bar and even visual options like showing macro text or hiding/showing the bar if certain circumstances are met (for example upon combat or when your pet is active). Many neat things can be done with this as long as your mind is creative enough.
Commands:/bt - Opens main configuration
/kb - Allows for quick keybinding. Can also be accessed through main configuration.
/bf - Allows re-skinning of buttons (requires Masque/ButtonFacade)