Email on Acid offers a free template with a basic, fluid design that's also responsive to mobile devices. In other words, the three different "layouts" you see below trigger based on the width of the recipient's screen.
Although there's only one template here, you can actually mix and match each section of the layout to fit your specific design needs. The layout supports one, two, or three columns, and recipients on mobile devices will see the version that converts to a one-column layout for easy reading.