That is one of Xon's awesome add-ons. It brings new features like "similar threads" and one of the best features is the "search bar auto complete"-function which really brings up the results you need when you search for something. Many options to configure and well-thought-out.
Great developer and very responsive for suggestions.
I have been working with @xon on getting the XF2 version to match the XF1 version when it comes to relevant threads, I can only say he has been extremely professional, receptive and prompt in providing me versions to test on my test forum. Also this addon is worth its weight in gold :)