Voice Search is a Google Chrome extension that lets you search using your voice. It's not developed by Google, but it uses an experimental Chrome feature called form speech input. The feature is enabled by default in the dev channel builds, but it can be manually enabled by adding a command-line flag.
"Voice Search comes pre-loaded with the following default services: Google, Wikipedia, YouTube, Bing, Yahoo, DuckDuckGo and Wolfram|Alpha. You can also add your own user-defined search engines. It also integrates a speech input button for all websites using HTML5 search boxes. This extension requires a microphone. Speech input is very experimental, so don't be surprised if it doesn't work. Also, try to speak clearly for best speech recognition results," suggests the author.
Speech recognition is limited to English and it doesn't work very well, but this extension is a good way to test a feature that will be enabled in the future Chrome releases. If you have a website, it's quite easy to add support for speech input, but it may take a while until Google's Speech Input API specification becomes a standard and all browsers implement it.