If you are already using Elementary OS Luna then you don’t need any introduction about wingpanel. But if you are not using eOS, then it is a top panel, similar to one you had in gnome 2 which displayed indicators and date/time.
But in this new slim wingplanel version, developed by Heath Paddock the panel’s width is reduced only to display the indicator icons and hovers over windows, hence maximizing the available vertical pixels for content as you can see in the below image.
You can insatll this slim wingpanel in eOS luna by using following commands
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The only downside of this is that there is no icon/link to launch slingshot. But you can either use super+spacebar keyboard shortcut to launch slingshot or install synapse search search indicator. Use instructions on this site to install synapse search indicator and you can launch it using super+alt+spacebar key combinations.
Below is a screenshot of synapse search indicator in action.