From a post by Dan Phiffer, an image by Adam Nieman and the Science Photo Library.
All the water in the world (1.4087 billion cubic kilometers of it) including sea water, ice, lakes, rivers, ground water, clouds, etc. Right: All the air in the atmosphere (5140 trillion tonnes of it) gathered into a ball at sea-level density. Shown on the same scale as the Earth.
More information at the original post. (Thanks to Eugene for the link.)