TotalFinder is probably.. no wait! It is the single most useful addon application for OS X!
BinaryAge finally brings the joy and necessity of tabs to Finder, just the way it should be. The single most important feature you've been missing. Thankfully, Apple's reluctance to properly develop Finder, has resulted in this nifty little addon application that I would gladly label a significant, albeit, small revolution for all Mac OS X users.
Say goodbye to multiple Finder windows and say hello to tabs (as many as you like) and the dual-mode view which is nothing short of ingenious. Using Finder has never been more fun or productive. The good people at BinaryAge surely deserve all the attention and apprisal they can get. Kudos!