Chrome (Beta/Dev): Voice input is now baked into Chrome's beta version and soon to hit the standard, stable release. When it does, consider installing Speechify. This simple, handy extension adds a microphone icon to Google, Amazon and other sites, so you can simply say what you're searching for.Speechify adds its icon to Google.com, in particular, and with the power of Google Instant, that means you can search Google without ever having to hit Enter. How well Speechify works for you depends on your computer's mic, as well as your ability to speak clearly at a fairly rapid clip. For those who don't mind talking to themselves, though, it could be rather convenient.
Speechify is a free download for Chrome's Beta and Dev channels at the moment, and will soon work in Stable. (Head here to update to a more bleeding-edge Chrome).