From Wikipedia’s list explaining the origins (some disputed) of company names:
Google – the name started as a jokey boast about the amount of information the search-engine would be able to search. It was originally named ‘Googol’, a word for the number represented by 1 followed by 100 zeros. The word was originally invented by a 9-year old boy Milton Sirotta, nephew of mathematician Edward Kasner in 1938 during a discussion of large numbers and exponential notation. After foundersâ€”Stanford grad students Sergey Brin and Larry Pageâ€”presented their project to an angel investor, they received a check made out to ‘Google’!
Link via Language Hat.