This simple site is incredible. (Update: The twins have made their incredible site an app that can be purchased through the app store, but a viable alternative is available here.) My sombrero (galaxy) goes off to Cary and Michael Huang for this elegant visual reference of literally everything in the observable universe. Beginning at human scales you can investigate smaller and smaller. You can fly past “smallest thing visible to the naked eye”, past cells and viruses, molecules and sub-atomic particles, right down to a representation of Planck Length. On the big side we have things like Central Park, Mount Everest, moons of our solar system, planets, stars, total human height (I know, right?), a light year, nebulae, galaxies and eventually the whole shebang that science (or physics, perhaps) has allowed us to see.
What makes this mind-blowing visualization even better is the fact that you can click on everything and get a brief description. I see the Great Barrier Reef hanging out there next to the moon, but how big is it? One click tells me that it is 2600 kilometres and that it is the biggest thing made by living organisms. The Great Wall floating nearby is giving it a good run for its money, though.