Google has launched voice search for Google.com search engine. This was announced by Google in June 2011 and would be coming soon. It would only be supported by Google Chrome browser. Before this Google has provided this voice service in some devices like voice search to the BlackBerry Pearl version of Google Maps for mobile, Google Mobile app for Blackberry and Nokia (Symbian) mobiles that allows users to search Google by voice. However now it would be offered for everyone for Google search engine where Queries can be search just by click and voice. It would be supported in many languages. And also for this service you need an attached microphone which can input your voice.You can get this Google chrome app here. Also please note that this is not 100 percent correct and may have minor problems but overall its really a cool app.
Check this out to get more idea on how it works.