I have come across three resources on this topic lately:
The first is a swf from an Australian e-card company. I believe this was the song sung to convince a woman to give up her liver in Monty Python's Meaning of Life.
The second is a fascinating set of images meant to give perspective to numbers like
one million plastic cups; the number used on airline flights in the US every six hours.
The third is the Power of 10 video, which many of us remember watching in Science class. The field of view expands out to the universe and then back in to the atomic level.
Do you have any favorite resources to give students a sense of scale?