I've been planning on reevaluating my add-ons, deleting those that I don't use, and adding those that are highly recommended. (Then I found out that invasions were useful for leveling alts, and well, there went three weeks of my time. :-)
One that I see recommended a lot (on Icy Veins) is ElvUI. I THINK it's a bartender-style add-on, right?
I am trying to find the base add-on in Curse, but all I see are a lot of ElvUI plug-ins.
ElvUI is a user interface suite, which aims to replace the default Blizzard UI on every level from buffs to action bars to the chat box. it does more than just handle your action bars like Bartender does.