There is, however, one name that is often linked to some of the key changes in spelling and usage which made American English distinctive, and it's that of Noah Webster (1758-1853). Webster was a schoolmaster who, like many such men of his time, wrote his own textbooks, with a focus on orthography -- the spelling and pronunciation of words -- as well as elements of grammar. He used the books in his school, and sold them to others; at a time when the printing business was in its infancy, Webster's "Spellers" as they became known, soon dominated the market. The money he earned from them enabled Webster to work on his Dictionary, on which his larger hopes and ambitions rested. In his instruction, as in his books, Webster held that the study of Latin and Greek, admirable though it was, should not take precedence over the study of English, and that everyday American usage, not that of British aristocrats, should be the model. He once lamented that "the whispers of common sense, in favour of our native tongue, have been silenced amidst the clamour of pedantry in favour of Greek and Latin."
One may note his usage there -- clamour and favour used the older British spellings that Webster would eventually abandon, preferring clamor and favor -- but his was no overnight reform, and the changes he made took place gradually over the course of his career. Perhaps that, along with the ubiquity of his books, helped American adapt to, and eventually adopt, his spellings. Not all of his reforms succeeded -- we still use ache instead of Webster's ake, soup rather than his soop, and tongue rather than tung -- but for the most part, his changes have become permanent. He even had some influence on British usage -- Charles Dickens, for one, tended to use the -or rather than the -our endings -- but his publishers corrected these and so the Brits (and Canadians and Australians) are stuck with with them.
American English also marked a period of renewed growth and change in our lexicon. We added words from native languages (moccasin, kayak, tomahawk), as well as from the waves of immigrants who arrived on our shores (look up brogan, schamltz, gauntlet, banana, tycoon, paprika, mammoth, or adobe for a sense of how wide the influence has been), along with all kinds of crazy coinages of our own, from shenanigans to shindig, from oink to woof, from pesky to phony, from vittles to vroom. Much like the UK, strong regional variations of English have developed, not only in terms of phonology but also lexicon and idiomatic phrases; the Dictionary of American Regional English catalogs these, and there are tens of thousands of them. Would you eat a "dropped egg"? How would you feel if you had the "mulligrubs"? And what does one do in a "chic shack?" All things considered, the American turn of English has been a wonderfully enriching one, and it ain't done yet.