AFAIK, Apple first introduced lightning fast desktop search in Mac OS X Spotlight. Being both a Mac and linux user, Spotlight is one of the tools that I use the most - to search for documents and more importantly, to launch applications without searching for its icon and/or name.

Linux comes with Beagle, a tool written on Mono that behaves like Spotlight.

A new tool was developed by ETRI, a South Korean R&D firm, and called "antbear". ZDNet Korea writes about the new Linux tool.

More tools competing against each other is good for consumers. Consumers will surely get choices and developers will always strive for better products. I just hope that antbear does away with Mono. :)