Cell Size and Scale is a little Flash learning aid from the University of Utah. It gives the viewer a good sense of the relative size of things, starting with a sheet of paper with a line printed in 12-point font, and shrinking all the way down to a single carbon atom.

Along the way you will find an HIV virus (130 nm -- the same size as an influenza virus), E. coli bacterium (3 x 0.6um), a single sperm cell (60 x 5um), and other such tiny critters. Zooming is quite smooth, and it's a great way to understand what's bigger than what, and how tiny some things really are.
Link: Cell Size and Scale