American Heritage Dictionary
(1)
Century Dictionary
GNU Webster's 1913
WordNet
(2)
Elsewhere on the web
It doesn't reflect unemployment, housing starts, inflation or a zillion other indicators.— Drudge Retort
Facebook's developer platform has been used for a zillion marketing campaigns so far, but this one is actually dead-on hilarious.— Original Signal - Transmitting Buzz
We have, of course, been whining about the quality problems of zillion-megapixel cams for a while now.— Wired Top Stories
No - one in a billion, one in a trillion, one in a zillion (and that's no exaggeration!).— Roni's Weigh
We've had Railroad Rush Hour for a zillion years and used to be completely obsessed with it, but then it entered the black hole known as the Game Closet …— PW FULL RSS FEED

American Heritage Dictionary (1)
Bubble size: how much this word was used in a year
Bubble height: used more or less than expected, vs. all uses evenly distributed
You can expect to see this word several times a year.