Using the Chrome speech API, dictation.io is a quick and simple speech to text web app.
Requiring no registration, dictation.io works a lot like Voice Typing in Google Docs. Click the START DICTATION button and start talking. The document is saved in the browser, but I could see this being used to enter text into the computer before copying it into another app, such as Gmail.
Dictation supports all of the languages available in Chrome, and requires the Chrome browser