Google Desktop 4 is now out of beta status. It gives you easy access to information on your PC and from the web by providing full text search over your email, files, music, photos, chats, Gmail, web pages that you’ve viewed, and much more.
It makes searching your computer as easy as searching the web with Google.It also has numerous widgets created by google and other developers for various perposes like stock ticker, weather etc.It supports 26 languages at present.
Yahoo! also has similar stuff for desktop called Yahoo! Widget Engine.