X1 has made their enterprise desktop search software (which was earlier priced $75) completely free. X1 is a very fast and powerful desktop search client with many built in previewers. But I am sticking to Copernic Desktop Search 2.0 Beta which has always been free and is very lightweight on system resources. The best new features introduced in the Copernic beta version is real time search results (results appear “as you type”) and the ability to save favorite searches (”My Searches”)

X1 homepage
Copernic Desktop 2.0 Beta