Google desktop was launched some time ago for Windows and now (after 3 years) they came with a Linux version. For those who doesn't know what it is :
You have lots of information on your computer, and you see a lot more on the web. With Google Desktop, whenever you look at a web page, read an email or edit a file, Desktop indexes that item and copies its contents into your local cache, so you can quickly and easily find all this stuff right when you need it.
You can read more here.
There is also a paid version (enterprise) for searching company networks.
Anyway we are glad to have a Linux version ...
You can get it from here.