Best Desktop Search client ever — get it here: http://www.x1.com/download/ then click the link for the X1 Enterprise Client.
This is the same desktop client that Yahoo is using, except in my experience, this one is a bit more stable / less buggy.
The real power of X1 is how crazy fast it is. As your typing it’s returning instantaneous results. The content preview windows is also incredibly fast — you can actually view an Outlook message or the contents of a file faster in this preview window than you can by using the native application!
Try it with a PDF file for example. Very good, very fast, get it.