This is an awesome interactive flash animation, featured by Phil Plait on his blog Bad Astronomy, showing the relative size of everything in the Universe, from the Planck length and the quantum foam to the size of the entire Universe, and everything in between.
It's truly mind boggling being able to compare things of such disparate size. Even things I expect to be relatively close in size turn out to be much farther away. For instance, zoom in on the proton, then look for the three quarks that make it up. You see that tiny dot directly in the middle? That's them. The proton is almost completely empty space.**
Or take a look at the neutrino, the smallest particle at the 1 yoctometer mark (that's 10-24 meters), then scale down until you get to the Planck length and the quantum foam. You have to go 10 orders of magnitude down to get there (.0000000001 yoctometers!), with nothing in between the two. Truly amazing.
Go check it out, you'll be tested later.
**I think Lawrence Krauss said that 90% of the mass of the proton comes from quantum fluctuations in empty space in this awesome video on Cosmology.